Degree project work
Name: Henric Svensson Subject: Environmental science Level: D
Nr: 2010:M2
Leaching Test with Sawdust from
Different Tree Species
– Appropriateness of using them as adsorption
media in wastewater and in stormwater
– Appropriateness of using them as adsorption media in wastewater and in stormwater
treatment
Henric Svensson
Environmental Science 240 hp
Degree project work, Environmental Science
30 hp Master of Science
Supervisor:
William Hogland, Professor, Linnaeus University, Sweden
Co- Supervisors: Marcia Marques, Professor, Linnaeus University, UERJ Brazil
Examiner:
Pasi Peltola, PhD, Research associate, Linnaeus University, Sweden
Abstract
Bio energy in form of woodchips and sawdust is today commonly stored outdoors in heaps on hardened surfaces, exposed to weather and wind. Any water leaching from these heaps have the potential to be toxic to the environment.
This paper examines the quality of the water leaching from heaps of four different tree species (oak, pine, maple and beech), by analysing different parameters such as pH, conductivity, colour, COD, BOD7, tannins & lignins (T&L) and phenols.
The results show significant higher leaching values of COD, phenols, T&L and colour from oak compared to the other tree species (pine, maple and beech). These leached substances from woodchips and sawdust were shown in the BOD7 tests and BOD7/COD ratio values to be hard to biodegrade and are therefore not easily removed from the water.
Hence it is important that wood-based fuel storage conditions are considered in bio energy generation schemes to ensure that the environmental benefits of using woodchips and sawdust instead of traditional fuel are not offset by the potential harm of inappropriate storage.
The investigation further showed that leaching of highly toxic substances such as phenols can be up to 10 times higher for one tree type (oak) than another (pine, beech and maple). This difference could potentially be found for other tree species not characterized in this study. Therefore, it is important to consider the constitution of the heaps to be able to apply appropriate storage conditions to avoid these toxic substances in the leached water reaching sensitive watercourses. As some of these substances are hard to biodegrade the treatment applied need a long retention time.
Preface
This thesis is part of a Master of Science (one year) in Environmental Science, at the School of Pure and Applied Natural Sciences. It is the result of a sub-project within a larger scientific project developed at the University of Kalmar namely “Development of an Integrated Approach for Industrial Wastewater and Stormwater Management in the Wood-industry Sector” conducted by the Environmental Engineering Research Group which works on developing low cost, easy-to-operate and nature-based waste water treatment methods to reduce the contribution to the environmental pollution from the wood industry. The project seeks for solutions that use as much as possible waste fractions from the industry for the development of treatment systems. This sub project has been focused on the leachate from sawdust in contact with water under different solid: water ratios. Therefore this study addresses water-related issues of companies in the wood sector. Another Master of Science student has at the same time, investigated electrocoagulation technique as a possible pre or post-treatment option for wastewater and stormwater from the same industrial sector. This project is sponsored by the KK-Foundation (KK-Stiftelsen) in cooperation with four companies in the wood sector.
For many years the environmental focus has been on the large wood industries such as the pulp and paper mills (Ali et al. 2001; Thompson et al. 2001). For this kind of, many types of water treatment systems have been developed (Pokhrel et al. 2004). Now it is the time to change focus to the smaller and medium size wood manufacturing industries such as floor and furniture manufacturers, which do not have water as part of the manufacturing processes but do use water for washing and cleaning surfaces, machineries and floors, generating low-volume of highly polluted wastewater. It is therefore, the purpose of the main project to which this sub-project is connected, to find solutions that are economically attractive not only for large but also for the small and medium size companies. This sub-project has been focused on comparative leaching from sawdust of different tree species. In this study, the leachate from 4 different tree species (oak, pine, maple and beech) has been characterized for the following parameters: pH, conductivity, colour, COD, BOD7, tannin & lignin (T & L) and phenols. The selected test methods used in this thesis have been chosen for the following reasons:
To carry out a pre-study investigation about the appropriateness of using sawdust as an adsorption material in treatment schemes;
To use the knowledge gained about sawdust leaching to support ongoing work with developing system for stormwater and wastewater;
To contribute to the assessment of potential water pollution resulting from wood material in contact with water.
Since large heaps of sawdust and wood heaps, as well as log yards are stored outdoors, once in contact with rain and snowfall, these wood materials can potentially release a number of substances that pollute recipient water bodies.
Based on these objectives the discussion in this report is divided in two main chapters: one for the adsorption study and one for the stormwater treatment.
Sammanfattning (Summary in Swedish)
Denna avhandling är en del av en Master of Science (ett år) i miljövetenskap. Den är resultatet av ett delprojekt inom ett större vetenskapligt projekt vid Högskolan i Kalmar nämligen "Development of an Integrated Approach for Industrial Wastewater and Stormwater Management in the Wood-industry Sector". Projektet genomförs av en miljöteknisk forskargrupp och målet är att utveckla lågkostnad, enkel och naturbaserad hantering av processvatten för att minska miljöbelastningen från skogsindustrin, och om möjligt använda delar av avfallet från industrin i behandlingen av processvattnet. Detta delprojekt är fokuserat på lakvatten från sågspån vid olika L/S förhållande. Denna studie riktar sig speciellt till företag inom träsektorn och andra civilingenjörsstudenter som studera natur och miljövetenskap. Projektet stöds av KK-stiftelsen i samarbete med fyra företag inom träsektorn.
Under många år har fokus på miljöarbetet inom träindustrin legat på de storskaliga massa- och pappersbruken (Ali et al 2001, Thompson et al. 2001). För denna typ av industrier finns många olika system tillgängligt för vattenrening (Pokhrel et al 2004). Nu är det dags att byta fokus till mindre och medelstora aktörer inom träindustrin, såsom golv- och möbelindustrin. Dessa industrier har inte vatten som en del av tillverknings processen utananvänder vatten för rengöring av ytor och maskiner och golv. Dessa processer skapar låga volymer av kraftigt förorenade vatten. Det är därför det övergripande projektet har till syfte att finna lösningar som skulle vara ekonomiskt attraktiva för de små och medelstora företagen. Detta delprojekt har varit inriktat på att jämföra lakvatten från sågspån.
I denna rapport har lakvatten från fyra olika trädslag (ek, tall, lönn och bok) karakteriserats för följande parametrar: pH, konduktivitet, färg, COD, BOD7, tanniner & ligniner och fenoler. Syftet med de valda testmetoderna i denna avhandling har valts efter följande anledningar:
För att utgöra en förstudie och undersöka användbarheten av sågspån som ett adsorptionsmaterial.
För att använda kunskapen i pågående arbete med att utveckla system för dagvatten och avloppsvatten.
Att bidra till bedömning av eventuella vattenföroreningar orsakade av trämaterial i kontakt med vatten.
Eftersom stora högar med sågspån och trädrester lagrars utomhus kommer dessa i kontakt med regn vid denna kontakt lakas det ut ett antal ämnen som förorenar mottagare vattenförekomster.
Baserat på dessa mål är diskussionen i denna rapport är uppdelad i två kapitel: en för adsorptionsstudien och en för dagvattenbehandling.
Contents
Terms and Acronyms ... 1
1 Introduction ... 3
1.1 Pollutants generated in the wood industry... 4
2 Objectives ... 6
3 Material and methods ... 7
3.1 Sawdust preparation ... 7
3.2 Methods ... 8
3.3 Chemical analyses ... 10
4 Results ... 11
4.1 Units ... 11
4.2 Results of chemical analyses ... 11
4.3 Solid to liquid ratio (S/L) ... 17
4.4 Batch leaching over time ... 17
4.5 Repeated washing of batches ... 17
4.6 Testing the effect of metals on colour of leachate ... 18
4.7 Effect of contact time on leaching according to studied variables ... 20
4.8 Simulating the relative contribution of sawdust from different species ... 22
5. Discussions ... 24
5.1 Stormwater ... 25
5.2 Effects of metals on the leachate colour ... 25
5.3 Proposal of wetland for leachate treatment ... 26
5.4 Proposal for further research ... 26
Conclusions ... 27
Acknowledgements ... 28
List of Figures ... 29
1
Terms and Acronyms
COD Chemical oxygen demand
BOD Biochemical oxygen demand
L/S Liquid solid ratio
DOC The fraction of organic particles below 0.45µm T&L Tannins and lignins
Grey-water The water from dish washing, laundry and bathing.
Leachate The extracted substances dissolved in a liquid medium, from a solid material. Wood debris The unsorted rest products of wood handling included sawdust and wood chips. Wood chips Pieces of wood (bigger than sawdust) chopped to pieces. Up to 10 cm
Sawdust Pieces of wood up to 4 mm.
Resins acids Toxic compound found in the wood, produced by the tree to act as a protective and preservative substance.
Hypoxia Reduced of dissolved oxygen content of a body of water to a level detrimental to aerobic organisms.
3
1
Introduction
The wood-industry, and in particular the paper and pulp industry, is often associated with large scale pollution of various water courses and coastal areas (Ali et al 2001; Thompson et al. 2001). A considerable amount of research applicable to this industry has been conducted during the years to try to find processes for pulp mills to only use renewable materials and make good use of all output materials from the mills i.e. reducing waste production. For example, between 1996 and 2002 MISTRA (The Foundation for Strategic Environmental Research) conducted a project called KAM - The Ecocyclic Pulp Mill, costing over 100 million SEK. As a result of this and other factors nowadays there exist high technological treatment systems available for these types industries. However, for the smaller manufacturers within the wood industry, such as furniture and floor manufacturers that do not have water in the manufacturing processes but generate low volume of highly polluted wastewaters, much less investigations have been carried out, mainly due to lack of focus and investments for this type of wastewater. These companies have a need for low cost and easy-to-operate technologies that can be adapted locally. Today there are few conventional functional systems available for the treatment of small quantities of wastewaters for their specific needs. Although the generated volume can be small, the wastewaters can have high concentrations of various constituents which in many cases can contain very industry-specific contaminants. Thus, there is a need to further development of wastewater treatment options for such purposes. In south-eastern Sweden one of these companies is the wooden floor manufacturer AB Gustaf Kähr in Nybro. This company processes annually about 83000 ton of raw wood. During the processing of wood to produce wood floors, it generates several small but highly polluted wastewater streams. Today, these wastewaters are left untreated and are discharged into the municipal sewage net or the stormwater network, thus potentially either causing direct harm to the local environment or adding load to the communal wastewater treatment plant.
The aim of the main large project as a whole is to develop water cleaning systems that implement integrated waste water management including process water, stormwater and perhaps also leachate from their landfills, located within the factory area. The aim of this sub-project is to evaluate sawdust as an adsorbent material to be used in wastewater treatment schemes.
Sawdust has successfully been used as an adsorbent for many different types of pollutants in earlier studies, as for instance, textile dyes (Izadyar et al 2007; Dulman et al 2009) and metals (Acemioglu et al. 2004; Ahmad et al. 2005; Ajmal et al. 1998; Kaczala et al. 2009). Using sawdust to remove metals may have as a side effect, which is the COD raising. In this project, in particular, it is of importance that the COD concentration does not raise as a result of the sawdust usage. Here, increased COD could be detrimental to small recipient water bodies and perhaps outweigh the benefit of removing specific organic substances it is intended to be used for. Thus, before using sawdust as an adsorbent in these settings, its own leachate must be characterised. The industry in question uses many wood species and the remainders of the production, such as sawdust, are available to be used as adsorbents.
4 debris and sawdust) outdoors. Precipitation (rainfall and snowmelt) as well as irrigation water, used to avoid cracks and to protect against pests, causes leachate. Toxic concentrations of for instance phenols, T&L, resin acids, terpenes and for some wood species a low pH in the leachate have been reported (Hedmark et al. 2008; Tao et al. 2005; Rupar et al. 2004). COD is a proxy indicator that indirectly quantifies the amount of chemically oxidizing substances in the leachate. The main problem with leachate from wood industries is that high COD can create oxygen depletion in water courses and in addition contains some toxic organic substances (Hedman 2008).
The runoff has traditionally been assumed to be clean and it has been discharged untreated to the nearest recipient such as forest ditches or rivers. Another problem is the colour of the runoff water that can have an impact on the photosynthesis in the watercourse. This colouring is probably cause by T&L and humic acids (Tao et al. 2005).
Figure 1 illustrates how stormwater or snow melt come in contact with storage areas for sawdust, wood chips and log yards and impervious surfaces creating a water pollutant transport problem.
Figure 1: Stormwater from a storage area for oak wood chips at Kährs and stormwater runoff and collection in a downstream pond
1.1 Pollutants generated in the wood industry
5 between 30 000 – 150 000 mg/l have been reported for different process streams (Kaczala, 2009). The leachate obtained from the sawdust is in the error range of detection limits for the process waters (Kaczala, 2009).
BOD: “Biochemical Oxygen Demand” is a way to measure the amount of oxygen uptake, under a limited time by microbial activity. The idea is that a sample with a determined amount of oxygen is kept in a dark place for 5 days (BOD5) or 7 days (BOD7), which is the most common time period used in measurements in Sweden. At the end of the period the amount of oxygen is determined again and the difference in oxygen content in the water before and after is calculated. The BOD value gives a relatively good indication of how easy the organic matter in the water is biodegraded. However, in many cases 5 or 7 days are too short incubation periods to degrade many substances. Even though, this test gives a good indication of how difficult it is. In order to determine the degradation capacity in a wastewater the ratio BOD/COD often is calculated. A high ratio indicates easily degradable water and a low value shows that it’s hard to biodegrade the organic compounds in the water.
Phenols: All phenols are characterized by a common functional group, the phenolic hydroxyl group (Asger et al. 2008). This compound is water-soluble and acidic. Many of the phenolic compounds found in wood are there due to their micro-biocide and insecticidal properties (Samis 1999). This might explain why the phenolic compound constitutes the most toxic compound found in wood leachate (Goudey 1992).
Tannins & Lignins (T& L); are substances which comprise one or more phenol functional groups. They are common in plants and have different functions; Lignin Is the substance that joins the wood cells together (Samis 1999), whereas tannins function as a defence to stop fungi and bacteria attacking the wood (Chung et al. 1998). Both tannins and lignin are hard to biodegrade (Tao et al. 2005).
6
2 Objectives
The main objective for this thesis is to investigate and carry out a pre-study to characterise the leachate from sawdust from four tree species in order to give basic information about the potential the sawdust from these trees have as an adsorption material and to look at the leachate impact on the stormwater from outdoor wood areas.
Due to time constrictions the adsorption test itself was not conducted. Knowledge of how sawdust reacts in a liquid phase is required to use sawdust as an adsorption material. The basic questions to be answered are: What happens when sawdust is mixed with water? To gain knowledge about this, leaching tests of sawdust from different tree species were made. To support and gain pre-required knowledge on this subject, a literature survey was conducted. The leaching study contributed to the large project, since one of the contaminants in the industry’s stormwater is the leachate from wood debris stored outdoors.
The following sub-goals are included in the project:
to investigate the leaching from sawdust and characterize the leachate; To investigate the effect of different L/S ratios;
To investigate the effect of sawdust particle size; To investigate the effect of exposure time;
To investigate if there are any difference in leachate composition and quantity between different tree types;
To suggest a treatment system for stormwater.
7
3 Material and methods
In this project, sawdust from oak (Quercus robur), maple (Acer platanoides), pine (Pinus sylvestris) and beech (Fagus sylvatica) were used. The definition of sawdust in this project is industrial wood debris with a maximum size of 4 mm. Wood chips are larger and are produced using a chipper that grinds wood to sizes up to 10 cm.
The sawdust used in the tests was collected from a single tree (plank) from each one of the four species; it was not known where the trees had been grown. There are most likely differences in the quality of the sawdust related to where the tree has lived, due to local and geological conditions and the age of the tree when it was harvested. If these variables result in significant differences it is not known and it was not investigated during this study, although the hypothesis should be raised. 3.1 Sawdust preparation
The sawdust used in this report was collected from AB Gustaf Kähr factory in Nybro, Sweden. In order to homogenize the sawdust and ensure the particle sizes were below 4 mm, which is a requirement to apply the standard method used for this study (test 1 “Batch leaching over 24 hours”), the sawdust was sieved through a sieve of 4 mm grid.
Then, one portion of the sawdust is used to calculate the moisture content in the sawdust. This is carried out by drying the sawdust in an oven during approx. 24 h at 105 ˚C.
The sawdust used in the tests is not dried in the oven but the moisture content is accounted for as detailed in the standard.
8 3.2 Methods
The overall research diagram is presented in Figure 2.
3.2.1 Batch leaching test over 24 hours
This is a leaching test (Figure 3) based on the standard method (SS 12457-2:2002) with small modifications. Because of the absorption capacity of sawdust, to work with the liquid solid ratio 10:1 which is prescribed in the standard method was not feasible. Instead, higher liquid solid ratios 20:1 and 40:1 were used in this test. The liquid/solid ratio is calculated using the following adjusted equation to calculate the amount of water:
L20=20-(MC/100)*Md L40=40-(MC/100)*Md Where:
MC= moisture content (%)
Md= dry mass of the test portion (Kg)
L= amount of water to add for L/S 20:1 and 40:1
Figure 3: Picture of the setup of sawdust leaching test.
Selecting the tree species for the tests & obtaining the material
Batch leaching test over 24 hours
Washing of the sawdust (as
pre-treatment)
Batch leaching test with sampling over
time
Testing the effect s of metals on the colour
of the leachate
Data analysis.
9 The amount of sawdust used in each test was 0.009 Kg (dry weight) sawdust and then, mixed with distilled water or rainwater according to the equation in the standard method. The agitation was conducted using magnetic stirrers for 24 h. After 24 h the test portion was filtrated over a GF/C Whatman 0.45 µm microfiber filter. The analyses1 were then conducted on the filtrate. This test was repeated with rainwater instead of distilled water. The rainwater was collected as rainwater from a sheet metal roof located next to The School of Pure & Applied Natural Sciences, in Kalmar, Sweden about 30 km south east from Kalmar. It was stored refrigerated in 6 ˚C before use. The pH was 5.3 and the conductivity 35 µS/m COD, value too low to be analysed with available equipment.
3.2.2 Sawdust washing as pre-treatment
To establish the sawdust properties after washing, a series of tests were conducted with sawdust that was reused after the 24 hours test. In this case, the sawdust was treated exactly as in “4.1 Batch leaching test over 24 hours” but a larger amount of sawdust was used (0.018kg). After the test the sawdust was sieved with a 125 µm sieve. The sawdust was then dried in the oven before being used in a new 24 h leaching test. A smaller amount (average lost 1 mg each turn) than specified was used due to the fact that some of the sawdust was lost in the sieving procedure. The L/S properties were recalculated so that the L/S ration remained constant. This procedure was repeated to a total amount of 6 times. After that, the leaching concentration from the sawdust was too low for the analysing method used.
3.2.3 Batch leaching test with sampling over time
This is a further development of the ”4.1 Batch leaching test over 24 hours”. In this test a higher L:S ratio of 1:40, was used (0.018kg dry weight sawdust). Then, a small sample with aprox. 20 mg of the liquid/solid mixture was extracted at different time intervals. The samples were extracted with a modified syringe. The sampling procedure was based on the principle that if the extracted sample has the same liquid: solid ratio as the batch where it is taken from, the L/S ratio will not change significantly in the batch. Therefore the syringe had a big inlet to also allow the larger particles to be collected. After the extraction, the test portion was filtrated with 0.45 µm filters and directly afterwards the analyses were conducted on the sample, when it was not possible to make analyses on the same day the sample were stored in refrigerator to the next day and then analyzed. The pH and conductivity measurements were made directly in the beaker.
3.2.4 Testing the effect of metals on colouring of leachate
Test 1: Distilled water and sawdust from oak was mixed by shaking. In a E-flask 500 ml half-filled with sawdust (100 g), distilled water was added (500 ml) to a level so it covered the sawdust. Three batches were prepared of which, the first one contained only oak and water, the second one contained oak, Fe powder (about ½ teaspoon or 20 mg) and water;-and the last one contained oak, Fe (about ½ teaspoon), water and oxygen, which was added in form of compressed air.
10 Test 2: Separate batches of pine, oak, beech and maple mixed with Fe. The same procedure described in test 1 was applied.
Test 3: In order to find out if other metals than Fe would have the same effect as Fe, a batch using aluminium (Al) powder was prepared in the same way as “Test 2”, where instead of Fe powder, Al powder was used.
3.3 Chemical analyses
The analyses were carried out with at least 3 replicates and sometimes up to 6 replicates. The tests over time2 and the rain water test3 have only 2 replicates per tree species.
The analytical procedures described below were used for all tests.
The pH was analysed with a Mettler Toledo SG2. The pH was measured directly after filtration, under the “4.3 Batch leaching test with sampling over time” the pH was measured in the batch not in the filtrate. The pH is presented as the negative 10th logarithm of hydrogen ions concentration at 20 oC to the base 10 (pH).
Conductivity was analysed with a WTW Multi 342 and the results were presented as µS/m. The conductivity was measured directly after filtration, under the “4.3 Batch leaching test with sampling over time” the conductivity was measured in the batch not in the filtrate.
Colour was analysed with Lovibond daylight 2000 unit and the unit used is Pt/L. Dilution to high degree of the leachate was needed to get the samples inside the equipments operating range, which gives an uncertainty in the results (related to dilution effect and ocular reading of the colour). Phenols were analysed using Dr Lange kit4 (LCK 346, 5-200mg/L). In some cases, dilution was made to get the sample inside the operating range of the kit. The result was then recalculated and presented as mg/dry matter of sawdust.
To analyse the tannins and lignin’s the Standard Method “5550 tannin and lignin” (U.S Water Environment Federation.) was used. The results were then recalculated and presented as mg/dry matter sawdust.
Analyses of COD were made using a Dr Lange kit5 (LCK 114 150-1000mg/L). In some cases dilution was made to get the sample within the operating range of the kit. The result was then recalculated and presented as mg/dry matter sawdust.
Analyses were made according to the European Standard “ISO 5815:1989”. The bottle used was a 250 ml Winkler bottle. The result was then recalculated and presented as mg/dry matter sawdust. Seeding water was not used but it was assumed that the leachate itself contained a sufficient amount of biodegrading microbial organisms.
2
Se Methods ” Batch leaching test with sampling over time” 3
All the values when rainwater is used
4 Pre-prepared cuvettes test that is determining concentration using a spectrophotometer.
11
4 Results
4.1 Units
For this project it was decided to present the values as mg/kg dry mater according to the standard method (SS 12457-2:2002). Another way to present these results is as concentration (mg/l). In the last case the low L/S ratio will have the highest pollutant concentration values (Table 1). In the case the result is presented as the amount of leaching mg/kg dry matter then the high L/S will have the highest pollutant concentration values. This happens because more water in the high L/S ratio samples result in that even if the concentration is lower more pollutants have been leaching in to the solution, which is visible when data is presented in mg/kg dry matter. Therefore, it is always important to think of the unit selected when interpreting the data.
Table 1: Leachate quality with leaching tests using oak at different L/S ratios. Tree
type L/S
COD mg/l
mg/kg dry matter COD
Oak 40 2 365 94 600 Oak 40 2 340 93 600 Oak 40 2 170 86 841 Oak 20 4 060 81 200 Oak 20 4 310 86 200 Oak 20 4 100 82 077
4.2 Results of chemical analyses
Differences between oak and the other three species (pine, maple and beech) where clear shown for basically all parameters analysed. This shows that it is important to know what type of three species a storage heap consists of, to foresee the leachate that can be released from the heap, once in contact with water.
In order to get a fuller picture of leachate content and properties, each species have to be studied separately as it became clear each species have it very own properties. Statistical tests (two way ANOVA) show that there is a significant difference between leaching for all of the four tree types except between maple and beech.
pH: The leachate pH values for all four wood species under four different treatments are shown in
Figure 4. The liquid solid ratios tested seem to be of no significance for the pH of the leachate. However, the specie that generated the sawdust is clearly significant. This means that it might be possible to regulate/adjust pH in water for different purposes, with the use of different types of wood debris. However, the effect of this is not analysed and the buffering capacity of sawdust has to be tested.
12 Figure 4: The pH of maple, pine, oak and beech leachate after 24 hours leaching test at different solid/liquid (S/L) ratio, for distilled-, rainwater and after washing treatment. Result is shown with SD (number of independent observations: 4 > n <6).
Conductivity: The conductivity of the leachate does not differ much among different tree species. On the other hand there is a difference for the different solid to liquid ratios: S/L 1:20 and the 1:40 (Figure 5). This is probably because the dilution of the ions from the sawdust is bigger at the S/L ratio 1:40. All of the values found are below the levels in ordinary lakes in Sweden (average in Swedish lakes 200 to 2000 µS/M) which would imply that sawdust would have a low impact on conductivity in water (Bydén et al. 2003). This also indicates that the leachate does not contain high values of metals, or salts that should have increased the conductivity values.
Figure 5: The conductivity of leachate obtained from maple, pine, oak and beech after 24 h of leaching test at different S/L ratios, with distilled and rainwater. Bars show S.D. (number of independent observations: 4 > n <6).
Colour: The colour of the leachate is measured as Pt/L. This is not an exact way to measure the
colour and to get these waters within the range of the equipment capabilities high dilutions were made, which increase uncertainties. However, the difference in colour between oak and the other three species was easy to distinguishable (Figure 6). According to (Bydèn et al. 2003) a lake with colour values over 10 is considered coloured and with a value over 100 it is considered highly
3 4 5 6 7 8
Maple Pine Oak Beech
pH
pH
S/L 1:40 Washed S/L 1:40 S/L 1:30 Rain water S/L 1:20 S/L 1:20 Rain Water 3 53 103 153 203Beech Maple Oak Pine
13 coloured. Based on this classification, the leachates from all species studied were considered as highly coloured water.
Figure 6: Colour of maple, pine, oak and beech leachate after 24 hours of leaching test at different L/S ratios; bars show S.D. (number of independent observations: 4 > n <6).
COD: The differences are large when comparing the COD values of oak leachate with the leachate
from the other 3 species (beech, maple and pine). Oak had about five times the COD amount compared to the others. However, the oak’s wood chips do not leach the same amount in 24 h as the sawdust. The probable explanation is that it takes longer time for the water to penetrate the wood chips and wash out the substances and the contact surface is a lot lesser for the wood chips. The effect of the pre-washing is almost the same for both oak and beech, and gives approximately 70% less COD after washing (Figure 7). In Sweden there are no general threshold limits given for COD in stormwater according to the authorities. The limits and the level accepted is determined individually by the companies and the local environmental authorities. From 2009 on, the environmental authorities in Sweden have planned to establish limits for emission of this parameter, due to the EU Water Framework Directive-WFD (Vinrot, 2007).
0 500 1000 1500 2000 2500
Beech Maple Oak Pine
14 Figure 7: COD in leachate from maple, pine, oak, beech and oak chips (with and without pre-washing treatment) after 24 h of leaching test at different S/L ratios using distilled and rainwater. “Oak chips 48” are wood chips that have been allowed to leach previously for 48 hours. The error bars show S.D. and the number of independent observations is: 4 > n <6.
Phenols: Regarding phenols, oak gives about ten times more phenol substances in the sawdust leachate after 24 h of leaching test than the three other species. When pre-treatment of the material by washing it is carried out, only about 30% of the phenol compounds amounts are leaching from the sawdust for beech and oak (Figure 8). An interesting aspect observed is that the ratio for phenols and COD (Phenols/COD) seems to be almost the same for wood chips in the 24 hours and 48 hours test as for the sawdust 24 hours (Figure 6). Phenols are acute toxic to freshwater invertebrates such as Daphnia magna at levels ranging between 7mg/l to 200mg/l (WHO, Environmental Health Criteria 161, 1994). If it is assumed that the first leaching (24 hours) at this low L/S (20:1) is about 50%6 (Mclaughlan et al. 2009) of the total amount that could be released. Therefore one kilogram of dry oak releases about 6 000 mg of phenolic compounds. This means that on kilogram of oak sawdust can cause toxic effect in about 800 l of water, only taking into consideration, phenol content (therefore, without considering the T&L effect). Phenols stand for about 1-3% of the total COD content in the leachate (Figure 9).
6 See chapter “4.4 Washing”
0 20000 40000 60000 80000 100000
15 Figure 8. Phenols content in leachate from Maple, Pine, Oak, Beech and Oak chips (with and without pre-washing) after 24 h of leaching test at different S/L ratios with distilled water and rainwater. “Oak chips 48” are wood chips that have been leaching previously for 48 h. Error bars show S.D. The number of independent observations is: 4 > n <6.
Figure 9. Ratio of COD/phenols after 24 hours leaching test for beech, maple, oak, pine, and oak chip. Oak chips 48 are chips that have been allowed to leach previously for 48 h.
T&L: Tannins and lignin are analysed as one group of substances (Figure 10); it is therefore
impossible to find out how much tannins relatively to lignin is found in the samples using the selected method. However, there is one indicator which appears to be related to tannin respectively lignin content: tannins and iron react and give a black colour7 to the leachate. This indicates that a higher amount of the pine T&L is tannins than for beech and maple. Oak have a much higher amount of total T&L but the ratio of tannins respectively lignin cannot be distinguished (Figure 11).There is a potential problem with the type of analysis used in this study; the analysis of tannins & lignins is based on the colouring that appears when tannins & lignin react with folin phenol. However, according to Samis (1999) many other reducing compounds might also react with folin phenol giving
7
See chapter ” Testing the effect of metals on colouring of leachate “. 0
1000 2000 3000 4000
Beech Maple Oak Pine Oak chips
24 Oak chips 48
mg
/k
g
d
ryma
tt
er
Phenols
S/L 1:40 S/L 1:30 Rain water S/L 1:20 S/L 1:20 Rain water S/L 1:40 Washed 0,0 1,0 2,0 3,0 4,0Beech Maple Oak Pine Oak chips
16 the same colouring therefore, inflating test results. It has to be established that no other reducing compounds are present in the sample to have a correct correlation between T&L content and amount of folin phenol.
Figure 10. T&L content of maple, pine, oak, beech and oak chips leachate after 24 hours leaching test at different S/L ratio and after washing treatment. Oak chip 48 is chips that have been allowed to leach for 48 hours. Error bars show S.D. (number of independent observations: 4 > n <6).
Figure 11. T&L/COD ratio in leachate after 24 h of leaching test for beech, maple, oak, pine, and oak chip. Oak chip 48 is chips that have been allowed to leach for 48 hours.
BOD7: The BOD7 values are surprisingly low; only about 20% of the COD (Figure 12)This indicates that the organic substances in the samples were hardly biodegradable and/or it’s not enough time with 7 days (Bydèn et al. 2003). Especially phenol compounds and T&L celluloses are known as substances that are hard to biodegrade.
Figure 12: BOD/COD ratio in the sawdust leachate after 24 h of leaching from beech, maple, oak and pine at different S/L ratios.
0 10 000 20 000 30 000 40 000
Beech Maple Oak Pine Oak chips
24 Oak chips 48 m g/ kg d ry m att er
Tannins & Lignins
S/L 1:40 S/L 1:20 S/L 1:40 Washed 0,00 0,10 0,20 0,30
Beech Maple Oak Pine
BOD
/VOD
Ration COD/BOD
17 4.3 Solid to liquid ratio (S/L)
Based on Figure 13, there is no existing difference between the S/L ratios. However, if looking at Figure 6 is the difference very big. But this is the concentration in the leachate and that differs according to the S/L not to the total leaching. So the total leaching of substances doesn’t matter on the S/L ratio but the concentration of substances in the leachate does. However, at a bigger difference of the S/L ratio for example 1:200 a small difference in the total amount might exist. This because there might be an equilibrium relationship between the water and the sawdust and at a higher S/L ratio a greater amount could leach from the sawdust. This is not proved and only speculations.
Figure 13: The COD concentration of beech, maple, oak and pine for leachate after 24 hours leaching test at different S/L ratio. Error bar = SD; Number of independent observation: 4 < n > 6. 4.4 Batch leaching over time
The main result of this test is that after 24 h the equilibrium is reached for all water quality variables included in this study. No significant leaching happened after 24 h. This shows that beyond 24h, it’s the S/L ratio that is the most important factor for the quality of the leachate from the sawdust and not the contact time. The wobbling pattern found in the T&L (Figure 21) is probably related to the uncertainty in the test. If the test had been made on wood chips the graph would most likely show a longer contact time before the leaching would reach this equilibrium.
4.5 Repeated washing of batches
The profile of COD concentration after repeated washing of batches oak and beech sawdust with distilled water is presented in (Figure 14).
0 1000 2000 3000 4000 5000
Beech Maple Oak Pine Oak chips
18
COD (% of total leaching)
0 2 4 6 8 0 20 40 60 80 100 Oak Beech % No. of washes
Figure 14: The effect of washing sawdust repeatedly. No of independent observations: 4. The standard deviation (S.D.) were too small to be seem in the graph.
The experimental data showed that the COD concentration in the leachate decreased to about 40% after the second wash, which is similar to the effect previously described by Maclaughlan et al. (2008) who found that about 50% of COD released was removed by the first washing. COD concentration dramatically reduced after the second washing. High reduction of COD in the first washing might be due to the release of organic particles attached to the surface of sawdust and also some chemical substances (for instance, cellulose, T&L and phenol) from sawdust itself. However, the COD concentration was almost constant after the third washing and onward. The final COD concentration contained in leachate was less than 5% of initial COD after washed 6 times.
4.6 Testing the effect of metals on colour of leachate
19 Figure 15: The colour after mixing sawdust with Iron (Fe), batch: (1) oak; (2), oak and Fe; (3) oak and Fe and O2.
20 Figure 17: The mixing of different tree species and aluminium, (Al), batch (1), pine; (2), pine and Al; (3) oak; (4) oak and Al; (5) maple and Al; (6) beech and Al.
4.7 Effect of contact time on leaching according to studied variables
The effect of the contact time on the values of pH, phenols, COD and T&L (tannins and lignin) is shown in Figures 18, 19, 20 and 21. Excluding the variations registered during the first hours, with a trend for increasing concentration of phenols, COD and T&L and decreasing pH during the first 24 hours, no fluctuation of importance was observed after this contact period for any of the analysed parameters.
Figure 18: Leachate pH development over time during leaching tests with maple, pine, oak and beech sawdust and L/S 40:1. Error bars show S.D. (number of independent observations: 2).
3,5
4,5
5,5
6,5
0
24
48
72
96
120 144 168 192 216 240
pH
Hours
pH change over time
21 Figure 19: Phenol concentration over time during leaching tests with maple, pine, oak and beech sawdust and S/L 1:40 (logarithmic scale at the x-axis). Error bars show S.D. (number of independent observations: 2).
Figure 20 COD concentration over time during leaching tests with maple, pine, oak and beech sawdust and S/L 1:40. Error bars show S.D. (number of independent observations: 2).
22 Figure 21: T&L concentration over time during leaching tests with maple, pine, oak and beech sawdust, and S/L 1:40 (logarithmic scale at the x-axis). Error bars show S.D. (number of independent observations: 2).
4.8 Simulating the relative contribution of sawdust from different species
Putting the results of this study into the context of existing wastewater streams and measurement precision of analytical methods, one can calculate the amount of sawdust from different species that can be added to the wastewater with treatment purposes (removal of metals, for instance), without the side effect of increasing COD due to leaching from the sawdust itself.
For example, the glue water at Kährs (the wastewater generated after cleaning cleaning/washing of gluing machine) has about 25 000 mg/l of COD (Laohaprapanon, 2009). The COD concentration is measured with a Dr Lange kit (
LCK 114 150-1000mg/L
), which has a confidence interval of about 8% (± 4%). This means that for one cubic meter of the glue water analysed the error can be about 2,000,000 mg (2 Kg) of COD. The question is then how much sawdust is needed to affect the water quality significantly e.g. above the error (Figure 22). In this calculation it is assumed that the total leaching from one Kg of sawdust in the lower S/L ratio ratio (1:20) will be the double than the amount leached at S/L 1:20. This assumption is based on the observation previously mentioned that the first (after 24h) leaching is 50% of the total leaching from the sawdust8 (Mclaughlan et al, 2009). The results show that not before the use of about 12 kg of oak, and 30 kg of pine per m3 of water the leaching is in the size of the error of the analysing test for the glue water. This means that quite a lot of sawdust - almost 80 kg of beech and maple - could be used for adsorption without even affecting the waters in a notable way.
8 See chapter ”4.4 Washing”
1000
10000
0
24
48
72
96
120 144 168 192 216 240
m
g
T&L/kg
d
ry
m
at
ter
Hours
Tannins and lignins
23 Figure 22: The leaching from sawdust from four different tree types in one cubic meter of water. Based on at this low S/L the leaching is about double that the one at S/L 1:20.
24
5. Discussions
The two main goals in this project was to perform a preliminary study to assess the appropriateness of using sawdust as a low-cost absorbent material for removal of pollutants from industrial wastewater and; characterize selected compounds that are leaching from sawdust and their leaching patterns. A question raised was how to manage the stormwater mixed with the leachate leaching from sawdust heaps stored outdoors.
The main result from the tests is that there are big differences in the leachate composition generated by different tree species. Sawdust from oak, for instance, gives high values of the pollutants analyzed, compared to the other species. Most parameters analyzed (tannins & lignin, phenols, COD) are known to have negative impact on aquatic ecosystems. These results are not surprising, as oak is known as a hard wood that is resistant against degradation. The fact that fungi and bacteria cannot easily degrade oak might be explained by the high values of tannins and phenols (Samis et al, 1999) found in this type of wood and the low pH of these substances providing the trees with natural defence against microbial degradation. The high values of lignins (measured by high tannins and lignin content) would explain the hardness of oak wood. Lignins are the substance that keeps the cellulose cells together in the wood (Samis et al, 1999). For the other tree species, no big difference was found for these parameters, but for pH. The pH varied from 4.1 (oak) up to 6.8 (maple), which is also an interesting information from the environmental impact viewpoint, in particularly when considering the size of the receiving water body.
If it is true that high concentrations of T&L, phenol, etc are associated to high resistance to biodegradation, other resistant species such as European Larch (Larix decidua), Speckled Alder (Alnus incana) and Wych Elm (Ulmus glabra) (SLU, 2009), are expected to also give high values of pollutant in the leachate and attention shall be paid to the selected storage method. The knowledge gathered by this study could be useful when designing integrated wastewater treatment systems for the wood industry. It has been demonstrated, for instance, that sawdust from P. sylvestris shows a good efficiency as adsorbent to remove some toxic metals from wastewater (Kaczala et al, 2009). The sawdust from different species might also be useful for regulation of the pH in a wastewater stream. Flocculation of different pollutants is often pH dependent. The leaching from sawdust stored areas might be a good pH adjuster. The sawdust can also work as a filter and when it has been used the sawdust could be removed, dried and then burned as disposal method depending on the organically pollutants that have been flocked. This would have a further benefit gaining energy through the burning process.
In a trade-off approach, the adsorption of toxic substances must be better for the environment than the eventually negative effect caused by leaching of compounds from the sawdust. One constraint that might be raised regarding the use of sawdust as an adsorbent for treating different wastewaters is the risk of leaching of other pollutants and increasing COD concentration in the water. The simulation of this phenomenon using a real wastewater composition (glue water) from Kährs demonstrated that this is not the case.
25 One real problem is that the leachate from the sawdust of all studied species is not easily biodegradable (which is the case of wastewaters with BOD/COD ratio below 0.3 as stated by Bydèn et al. 2003 among others). Potential low-cost adsorbents studied during the last years are mainly straws, tree bark, peat and sawdust.Adsorption of metals by sawdust has been described by Shukla et al. (2002); Acemioglu et al. (2004); Ahmad et al. (2005); Kaczala et al. (2009); Ajmal et al. (1998). Sawdust has also been used to remove nitrogen from groundwater (Schipper et al. 1998). Removal of different types of dyes by sawdust has also been described (Hamdaoui, 2005). These small streams of contaminated water as for example the “glue water” consisting of about 500 l/week is highly contaminated with around COD 25 000 mg/l (Laohaprapanon, 2009). However it is not likely that sawdust will adsorb sufficient COD given substances from this kind of highly contaminated wastewaters.
5.1 Stormwater
Investigations have been carried out by the main project to which this sub-project is linked during more than one year when leaching from a field-scale storage heap of oak chip at Kährs in Nybro have been regularly monitored (Kaczala, 2009).
The runoff from the storage areas with wood debris and wood chips did not have as high values as at the leachate analysed in the present leaching lab study. This might be due the fact that the leaching from the field storage heap was probably diluted with stormwater runoff from surroundings before the sampling point. Even though, the concentrations found were high enough to require some kind of treatment before discharging the stormwater into recipient water bodies.
Since this stormwater has also low BOD/COD ratio, and therefore, low biodegradability, and considering the level of precipitation in the Nybro area in Sweden the design of on-site treatment sytem would require a big equalization pond for this water to be collected, which might not be a feasible solution for several reasons such as space and economics. The stormwater with the leachate from the heap studied in Nybro is currently discharged into a forest ditch that about 15 km downstream discharges into the “Natura 2000” classified river Ljungbyån. Therefore, it is of importance to find out the concentration and the total load of pollution in the water discharged into the ditch and also investigate the pollution situation at the point the water reaches the river Ljungbyån. It is likely that a dominating part of the pollutants already have been degraded or percolated into the groundwater along the river before it reaches the recipient river. One way to treat the runoff is to make a constructed wetland which would have a further benefit of increasing the biodiversity in the area. The wetland and the collection of the stormwater would require a quite large area on the industry area and also a new stormwater collection system. Another way to reduce the amount of leachate sufficiently which would also generate higher energy efficiency is to store the wood debris indoors. This might even reduce the risk of fire by self ignition through reducing the heat producing microbial activity in the storage of the heaps. This ought to be considered for storage of any oak, as oak releases very high amount of toxic compounds (phenol, T&L plus lowering pH).
5.2 Effects of metals on the leachate colour
26 5.3 Proposal of wetland for leachate treatment
Besides high COD content and low biodegradability, some studies point out high nutrients content (Jonsson et al, 2004) and others, high carbon content to nutrient as a constraint for treatment of these wastewaters in constructed wetlands (Tao et al 2004; Tao et al 2005). A BOD:N:P ratio of about 100:17:3 is recommended (Hammer et al. 2001). Studies have shown that a well adjusted BOD:N:P ratio raises the treatment efficiency of the wetland (Masbough et al. 2005). Masbough et al. (2005) investigated the removal of pollutants from stormwater generated from a log yard. They managed to remove BOD between 52-63% and COD between 25-51%. The study shows that adding nutrients (phosphorus and nitrogen as urea) successfully removed more pollutants from the stormwater than without. Low amount of nutrients can limit the organisms in the wetland to work efficiently (Gopal, 1999; Kadlec et al, 1996). In Masbough et al (2005) study is the oxygen level in the outlet of the treatment system very low (0.3mg/l) which indicates that the breakdown of organic matter mostly is done by anaerobic processes. This might suggest that if more oxygen is added to the system an even better uptake of COD might be reached inside the system.
Reduction of nitrogen and phosphorus is often the goal for constructed wetlands. This is usually done by combining anaerobic and aerobic conditions in wetlands. In the wood industrial sector which is the focus of the main project, the goal is to reduce the high COD. The problem would therefore be how to transfer oxygen to the water, which could be done by water overflow or by harvesting to stimulate biomass production through photosynthesis.
Thypha latifolia is suggested because it`s well-known as a resistant specie to heavy loads of pollutants (Masbough et al. 2005) but maybe in the later parts of the system, submerged plants and algae could be used to increase oxygen levels. To ensure optimal conditions for photosynthesis in the water shadowing should be avoided.
Another strategy to construct a wetland is to use an anaerobic wetland with high retention time (Hunter et al, 1993; Tao 2004).
5.4 Proposal for further research
One area of importance for further investigations is to clarify how aging affects the sawdust leaching pattern; as a result of microbiological activities in storage heaps.
Another practical aspect to clarify is related to the need of sawdust pre-treatment with washing or other treatment methods before using it as adsorbent, even when stored for long periods and leached by rainwater.
Investigation about the effectiveness of using sawdust to regulate the pH for different types of wastewater and the buffering capacity of sawdust leachate is also suggested. Furthermore, it would also valuable to know the leachate behaviour at different osmotic conditions that might occur in wastewater. Is the leaching the same when the water already have 20 000 mg/l in COD? Is there a chemical equilibrium function that makes the sawdust leaching less?
27
Conclusions
There is a significant difference in leachate quantity and composition of organically substances depending on the tree species.
Oak is leaching higher amounts of toxic substances to the environment than maple, beech and pine.
The high colouring of leachate from oak is likely to be caused by tannins reacting with metals in water, the metal probably being iron.
The leachate from wood is hard to biodegrade.
One kilogram of dry oak sawdust is leaching enough of phenolic substances to make 800 L of water toxic to aquatic organisms.
28
Acknowledgements
First of all I would like to acknowledge Professor William Hogland for giving me this opportunity to write my master thesis under his supervision, and helping to structure this thesis from a technical and scientific point of view. It has been very interesting to have the opportunity to work within a large research project, having close contact with other MSc students, PhD students, professors and students from different countries.
I know that I will have an enormous use of the knowledge gained concerning the management of large research projects in cooperation with the industry and international cooperation in teaching and research. I have learnt how to manage projects with the industry, in particular, how to prepare myself and the group for research group meetings, for meetings with the companies and meetings with international students, professors and scientists.
I also want to thank professor Marcia Marques together with professor Hogland for giving me the possibility to finalise the thesis and discuss my results with students and researchers in Brazil with the financial support from STINT (The Swedish Foundation for International Cooperation in Research and Higher Education) and for giving me valuable constructive criticisms on the report and assistance with statistical analyses.
Furthermore, I want to acknowledge the other research members of KK-Stiftelsen project for their support and, in particular the PhD students Mr Fabio Kaczala and Ms Sawanya Laohaprapanon for their help with the daily work in the laboratory and field studies as well as many cultural and sporty activities during our spare time.
29
List of Figures
Figure1: Stormwater from a storage area for oak wood chips at Kährs and stormwater runoff and
collection in a downstream pond ... 7
Figure 2: A conceptual research methodology...10
Figure 3: Picture of the setup of sawdust leaching test ... 10
Figure 4: The pH of maple, pine, oak and beech leachate after 24 hours leaching test ... 14
Figure 5: The conductivity of leachate obtained from maple, pine, oak and beech after 24 h of leaching test ... 14
Figure 6: Colour of maple, pine, oak and beech leachate after 24 hours of leaching test ... 15
Figure 7: COD in leachate from maple, pine, oak, beech and oak chips . ... 16
Figure 8: Phenols content in leachate from Maple, Pine, Oak, Beech and Oak chips ... 17
Figure 9: Ratio of COD/phenols after 24 hours leaching test ... 17
Figure 10: T&L content of maple, pine, oak, beech and oak chips leachate after 24 hours leaching test ... 18
Figure 11: T&L/COD ratio in leachate after 24 h of leaching test ... 18
Figure 12: BOD/COD ratio in the sawdust leachate after 24 h of leaching. ... 19
Figure 13: : The COD concentration of beech, maple, oak and pine for leachate after 24 hours leaching test. ... 19
Figure 14: The effect of washing sawdust repeatedly ... 20
Figure 15: The colour after mixing sawdust with Iron ... 21
Figure 16: The colour after different combinations of mixing sawdust and iron ... 21
Figure 17: The mixing of different tree species and aluminium ... 22
Figure 18: Leachate pH development over time during leaching tests ... 22
Figure 19: Phenol concentration over time during leaching tests ... 23
Figure 20: COD concentration over time during leaching tests ... 23
Figure 21: T&L concentration over time during leaching. ... 24
30
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Kalmar Växjö
