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Landfilling is one of the oldest and common methods used for waste disposal. It is perceived as the most economical and environmentally acceptable technique. It is a complex system with physical, chemical, and biological processes.
While undergoing the process of wastes degradation, there is the production of highly contaminating liquid, leachate, and polluting gases. If discharged in an uncontrolled and non-engineered manner, leachate will contaminate groundwater bodies and subsequently jeopardizing the ecosystem. There is a network for the collection the contaminants. The gases such as methane and carbon dioxide are flared before they can affect the atmosphere. The leachate generated, requires treatment before discharge and it is the main problem.
In Mauritius, there has been an upsurge in the amount of wastes generated due to rapid industrialization. A structure for solid waste management was necessitated which resulted in the construction of Mare Chicose Sanitary Landfill Site. Over the years, there has been an increase the volume of wastes being disposed and consequently, a rise in the amount of leachate generated.
As previously mentioned, the polluting liquid requires treatment prior to disposal. Nowadays, we do have laws that are regulated by the Wastewater Management Authority Act and the organization operates under the aegis of the Ministry of Public Utilities. After treatment leachate shall comply with the standard limits for effluent discharge as shown in Appendix C.
Many studies have been carried out for the treatment of leachate and various methods are available. There are several parameters that define the treatment method. The treating technique shall be efficient, cost-effective with minimum input, flexible and if possible usage of the effluent.
- Aim and Objectives
The aim of the project is the study of the nitrification process in the treatment of landfill leachate.
The project had the following objectives set:
- To determine the suitability and efficiency of a SBR and co treatment method for the treatment of landfill leachate.
- To find the concentration at which ammonia nitrogen is toxic to microorganisms.
- To design a suitable tank for the method being adopted.
- To assess the cost-effectiveness of the treating system
- Structure of Thesis
The remainder of this thesis is organized as follows:
Chapter 2: gives a brief overview of landfilling process, describing the various components of a landfill. There is a description of the Mare Chicose Sanitary Landfill Site and a summary of typical leachate effluent.
Chapter 3: deals with the treating options available for wastewater treatment particularly leachate. The efficiency for ammonia nitrogen removal is outlined and a reviewing some case studies on biological treatment of landfill leachate.
Chapter 4: describes the methodology adopted for leachate treatment.
Chapter 5: gives a detailed analysis of the results obtained and assessment of various parameters.
Chapter 6: consists of the design a treating system for leachate.
Chapter 7: describes the cost effectiveness of the treatment methods and some recommendations for improvement of the designs.
REVIEW of LITERATURE
A landfill may be defined as a physical facility used for the disposal of residual solid wastes in the surface soils of the earth (Tchobanoglous et al.). Nowadays, the term sanitary landfill is more usually utilized to describe an engineered facility, designed, operated and monitored with the foremost objective of reducing environmental and health hazards.
According to Tchobanoglous, a landfill may be categorized with respect to the incoming waste materials.
There are various criteria that are considered before the design and construction phases. The site cannot be close to water bodies, highways, any residential areas or even airports. The main reason is the pollution accompanied by the operation of such a site which will eventually disturb its surrounding environment.
Another factor is the hydrogeology of the site, groundwater maps are prepared by studying the different soil stratum. This helps in determining the permeability of the soil, the depth to groundwater, the direction of groundwater flow and hydraulic gradients. If clay is to be used as a liner, then borrow sources are found.
- Landfill Components
Liner: It is a barrier that will prevent the leachate and other liquids from penetrating the soil. It can be made of clay, synthetic materials or both which is known as composite liner. This barrier also restricts the underground migration of landfill gases.
Cap system: Usually a soil cover placed over the landfill at completion of filling, also known as final cover, with vegetation grown over it. The cover may consist of geosynthetic materials also, thus hindering the escape of landfill gases to the air and restricting the infiltration of rain into the landfill (Bagchi, 1994).
Gas management system: As shown in the diagram above, these are a series of gas wells that removes methane and other decomposition gases from the landfill for flaring and reuse. The methane gas may be used in the electricity production.
Leachate management system: A number of horizontal and vertical pipes placed just above the liner that drains and collects leachate. Afterwards the polluting liquid may be brought to a retention pond.
- Mare Chicose Sanitary Landfill Site
Over the last few years, a rapid development at socio-economic levels has brought an upsurge in the amount of wastes generated in Mauritius. There was a need for an integrated solid waste management programme.
The Mare Chicose Sanitary Landfill is the only waste disposal site for Mauritius till date. The site is located in the southern part of the island near a small village called Cluny. It receives mostly municipal solid wastes and therefore categorized as a Class Ш type.
The site was previously operated by STAM Ltée, from 1997 to 2006, and presently by Sotravic Limitée/ Bilfinger-Berger consortium. The amount of wastes disposed at the landfill has nearly tripled over the years, reaching to a daily value of about 1,200 tonnes.
The percentage of incoming wastes is summarized below:
The field capacity of the landfill was already attained and currently there is an extension of works on existing cells. The site is comprised of six cells and actually the fifth one is in use.
Prior to disposal at the landfill, the wastes are compacted at transfer stations. The wastes are dumped from a tipping point and soon, they are spread over existing wastes by means of specialized vehicles. At the end of the day, a cover is placed to reduce the amount of windblown debris.
Both clayey and geosynthetic liners were used on the site. The amount of leachate being carted away for the period of January 2007 - December 2007 is 110 858 m3. Actually, no leachate treatment is being carried out. Among the landfill gases produced methane is the most dangerous and it is dealt with in a controlled environment. The gas is being collected by means of pipelines and subsequently flared.
The definition according to EPA is as follows;
“Water that collects contaminants as it trickles through wastes, pesticides or fertilizers. Leaching may occur in farming areas, feedlots, and landfills, and may result in hazardous substances entering surface water, ground water, or soil.”
Leachate can be described as a highly contaminated liquid, containing a considerable amount of dissolved and suspended solids that has percolated down through wastes. The leachate quality varies throughout the operational life of a landfill and long after its closure. There are three broad and overlapping phases of waste decomposition, in which chemical and biological processes give rise to both landfill gas and leachate during and beyond the active life of the site (Carville et al.).
Phase 1: Oxygen present in the wastes is rapidly consumed by aerobic decomposition. This phase has duration of less than one month and is normally relatively unimportant in terms of leachate quality. This phase is exothermic and high temperatures may be produced. If some of this heat is retained, then as a result of that the rate of the upcoming phases is increased.
Phase 2: Anaerobic digestion is comprised of the following four phases;
- Hydrolysis: A chemical reaction where large polymers are converted to simple monomers.
- Acidogenesis: A biological reaction where the monomers are converted to volatile fatty acids.
- Acetogenesis: A biological reaction where the fatty acids are converted into hydrogen, carbon dioxide and acetic acid.
- Methanogenesis: The acetic acid is converted into acetates. Hydrogen is used up to convert the acetates into methane and carbon dioxide.
Anaerobic and facultative microorganisms hydrolyze cellulose and other putrescible materials such as complex carbohydrates, fats and proteins to soluble organic compounds. These hydrolysis products are then fermented during acidogenesis to various intermediates such as volatile fatty acids and alcohols. Finally, these intermediates are converted during acetogenesis to acetic acid, carbon dioxide and hydrogen. The high content of putrescible material in the waste may sustain acidogenic conditions for quite some time and provide a rich feed stock for methanogens subsequently. Leachate from this acidic phase typically contains a high concentration of free fatty acids. It therefore has low pH of 5 or 6, and will dissolve other components of the wastes, such as the alkaline earths and heavy metals, which can be mobilized in the leachate, possibly as fatty acid complexes. The leachate also contains high concentrations of ammoniacal nitrogen and has both a high organic carbon concentration and a biochemical oxygen demand (BOD).
Phase 3: Conditions become more anaerobic as waste degradation proceeds and methanogenic bacteria gradually become established. These start to consume the simple organic compounds, producing a mixture of carbon dioxide and methane that is released as landfill gas. The carbon dioxide tends to dissolve producing the very high bicarbonate concentrations typical of Phase 3 leachates. The rate at which this phase becomes established is controlled by a number of factors, including the content of readily putrescible waste. Since the majority of the organic compounds are high molecular weight humic and fulvic acids, the leachates are characterized by relatively low BOD values. Ammoniacal nitrogen continues to be released by areas of the waste where phase 2 is continuing and generally remains at high concentrations in the leachate. Falling redox potential immobilizes many metals as sulphides in the waste.
- Typical leachate effluent
Leachate is usually termed as a high strength wastewater. The polluting liquid has a high concentration of contaminants and varies throughout the landfill age as shown in the table below.
From the above table, it noticed that leachates are normally alkaline having a pH of 6.0-8.4. The average COD value is found to be 5000 mg/l and the ammoniacal nitrogen remains within a similar range 900-3000 mg/L for all most of the sites.
As it has been portrayed, the leachate does not meet the requirements for discharge either in sewers or surface water (see Appendix C) and this clearly indicates a need for treatment.
Most landfills operate their own onsite leachate pretreatment and treatment facilities. Three types of treatment are possible physical, chemical and biological. Usually they are used in conjunction with one another.
The constituents of leachate and availability of resources determine the treatment method to be adopted. Therefore, it should be efficient, flexible and an economical option.
The leachate quality is highly dependent on the waste materials being disposed and the stage of their anaerobic decomposition. Hence, there is a variation in the constituents' concentration.
It has been observed that throughout the life cycle of a landfill, the ammonia nitrogen concentration remains very high. Amongst several usual parameters, ammonia nitrogen is a key one as it influences the selection and the design of the treating system.
- Physical Treatment
- Ammonia Stripping
Ammonia can be removed by the air stripping technique which consists of blowing air through the wastewater. The method is based on the following equation;
The above equation is highly dependent on the pH so that an exchange of ionic forms can take place. The equilibrium constant for this reaction is 10-9.25 at 18° C (Sorensen, 1993).
pH = 9.25 + log [NH3] / [NH4+]
From the above equation a pH greater than 10 is needed for releasing the ammonia gas. At normal temperature only 2% of the gas is liberated and therefore the wastewater should be heated to increase the efficiency of the treatment process.
In achieving relatively low effluent values of ammoniacal-N (e.g. <50 mg/l), very large volumes of air will be required and this generally makes air stripping uncompetitive in cost terms for such applications The process is also inefficient in cold weather and requires shut down (IPCC, 2007).
- Reverse Osmosis
The process consists of applying a pressure to the wastewater, i.e. the leachate, which passes through a semi permeable membrane. The water molecules present in the wastewater will pass the membrane forming the permeate and the contaminants remaining are the concentrate.
The main advantage of using such a system is the removal of non-biodegradable compounds such as residual COD, heavy metals and chloride ions together with other large molecules present in leachate.
The concentrate produced is a major issue as it is highly toxic to the environment. It is usually recirculated in the landfill or disposed off-site for storage.
The removal rate of the contaminants is usually greater than 99.6 %. The plant is usually operated in more than one stage and occupies less space when compared to other treating systems. The process is currently in use in several countries such as France, Germany and Holland (IPCC, 2007).
- Activated Carbon Adsorption
Activated carbon is used as an adsorbent for the removal of organic compounds. It is used in one of the following forms, powdered and granular. Due to the high cost of activated carbon, it is normally utilized for polishing after biological treatment. With an optimum dose and sufficient contact time, a considerable decrease in COD and BOD concentration can be achieved by this method.
In the powdered form, the carbon is meant for single use and it loses its adsorption capacity and therefore cannot be reactivated. The mixed liquor must then be treated to remove the PAC, by subsequent processes, such as coagulation, flocculation, or filtration.
In the granular form, the carbon can be used again but must be removed which requires specialized equipment (IPCC, 2007).
- Biological Treatment Processes
The treatment process is comprised of growing and reproducing microorganisms in a controlled environment to stabilize organic matter. There are two forms of growth process attached and suspended. In suspended growth treatment systems, microorganisms are maintained in suspension within the wastewater whereas in the attached growth process, the biomass grows and is retained on a medium.
Attached Growth Processes
- Percolating filters
- Rotating biological Contactors (RBC)
Suspended Growth Processes
- Aerated lagoons
- Activated Sludge Process (ASP)
- Sequencing Batch Reactor (SBR)
Combined treatment with domestic wastewater (co treatment)
- Percolating Filters
It is an aerobic biological treatment system. Wastewater flows over a fixed and inert medium to which biofilms are attached and trickles down under gravity. The medium may be made up of different materials such as plastics and gravels and the depth of the filter is normally 2-4 m. The effluent is passed through a clarifier to remove biological solids.
The percolating filter has many disadvantages concerning the treatment of landfill leachate. The system is efficient mostly for the treatment of low strength leachate. A recurrent problem is the clogging of the filter media and vulnerability to shock-term load (IPCC, 2007).
- Rotating Biological Contactors
The process consists of large diameter steel or corrugated plastic media centered around a horizontal shaft, usually placed in a concrete tank. The media is slowly rotated (mechanical or air drive). At any given time during the rotation, about 40% of the media surface area is in the wastewater. Organisms in the wastewater are attached and, multiply on the rotating media until they form a thin layer of biomass.
RBC is most effective for treating methanogenic than acetogenic leachates and for concentrations of ammoniacal-N below 500mg/l. The rotating biological contactor may have operational problems, since high concentrations of degradable COD can result in excessive sludge growth, and clogging of interstices within rotors (IPCC, 2007).
- Aerated Lagoons
Aerated lagoons are operated by a combination of aerobic and anaerobic processes. The lower part of the lagoon converts the settled solids and sludge into carbon and methane by the action of anaerobic decomposition. The upper part is usually aerated, surface aeration or by algae present, to oxidize compounds from the anaerobic zone.
Effluent is withdrawn from the upper zone, generally over an overflow arrangement. For discharge into surface waters, a secondary settlement lagoon or reed bed filtration system is needed for wastewater polishing.
The constraints of the system are as such it requires large space and is quite sensitive to temperature changes. There is the possibility of odurs emanating from the lagoon. The main concern is the inability to provide consistent and reliable design in order to meet the discharge limits.
- Activated Sludge Process
It is the most widely used aerobic biological process for treatment of domestic wastewater. It operates on the basis of a continuous inflow of wastewater. The latter is completely mixed and aerated for certain period of time, giving rise to mixed liquor. For nitrification to occur the sludge age must be greater than 8 days, so that the nitrifying bacteria can grow sufficiently large in numbers to exert an oxygen demand. The mixed liquor is allowed to settle in the clarifier and the biomass is returned to the aeration tank. The clarified effluent is decanted for disposal or tertiary treatment. The ASP is a continuous process and leachate cannot be treated directly, it requires dilution due to ammonia toxicity.
- Sequencing Batch Reactor
The reactor is a slight modification of the ASP. It operates on a fill-and-draw basis using the suspended growth process. The SBR utilizes a single tank which accommodates aerobic biological treatment, flow equalization, settlement of solids, effluent clarification and decanting. Thus, it is usually described as operating in time rather than space when compared to conventional ASP.
The reactor consists and operates under the following cycles:
Fill: During the fill operation, volume and substrate (raw wastewater or primary effluent) are added to the reactor. The fill process typically allows the liquid level in the reactor to rise from 75% of capacity (at the end of idle period) to 100%. During fill, the reactor may be mixed only or mixed and aerated to promote biological reactions with the effluent wastewater.
React: During the react period, the biomass consumes the substrate under controlled environmental conditions.
Settle: Solids are allowed to separate from the liquid under quiescent conditions, resulting in a clarified supernatant that can be discharged as effluent.
Decant: Clarified effluent is removed during the decant period. Many types of decanting mechanisms can be used, with the most popular being floating or adjustable weirs.
Idle: An idle period is used in a multitank system to provide time for one reactor to complete its fill phase before switching to another unit. Because idle phase is not a necessary phase, it is sometimes omitted.
Advantages of the system
- It requires small space as a common tank is used for the various unit processes.
- Flexibility in operating the reactor.
- The reaction time can be controlled and settling can be achieved under quiescent conditions.
- There the elimination of the return sludge pumping when compared to the ASP.
Disadvantages of the system
- A higher level of sophistication is required (compared to conventional systems), especially for larger systems, of timing units and controls.
- Potential of discharging floating or settled sludge during the draw or decant phase with some SBR configurations.
- Combined Treatment with Domestic Wastewater
It is a combined method for treating domestic wastewater and landfill leachate. Both wastewater and leachate can be treated at suitable mixing ratios (Aktas, 2001). Domestic wastewater can provide phosphate while leachate can provide nitrogen based nutrients, thus compensating for nutrients deficiency. Hence, nutrients need not to be supplied.
Leachates from older landfills have a lower BOD/COD value and a smaller biodegradable organic fraction. There may not be sufficient COD to support denitrification of nitrate, a supplementary source of organic carbon is required to ensure adequate denitrification. Synthetic chemicals, such as methanol or acetic acid, are effective but quite expensive. It is necessary to find an alternative cost effective source of easily biodegradable carbon (Zhang, 2005).
The mixing ratios are determined or else there will be nitrification inhibition by the presence of excess free ammonia.
- Case studies for biological treatment of landfill leachate
The Buckden Landfill Site has been operational since 1994 and has been successful in treating landfill leachate for more than 10 years. The landfill site uses twin sequencing batch reactors, each designed for treating up to 100 m3/day. The effluent is then treated by means of reed bed and an ozonation plant for wastewater polishing and removal of pesticides.
The plant has a design loading rate of 0.02 - 0.040 kg N/kg MLVSS. The plant has been successful in removing ammonia nitrogen from 331 mg/L to 0.27 mg/L. Only the COD value has not met the discharge limits (< 100 mg/L) and the COD reduction was from 843 to 320 mg/L. However, the COD value was acceptable since leachate is usually comprised by high amount of inert fractions.
The main running costs are due to electricity for aeration and for ozonation. There is also the use of sodium hydroxide for automatic pH control, and of phosphoric acid for provision of phosphorus as a nutrient, which are relatively small costs.
Another case is a South-African landfill which receives up to 2000 tonnes of MSW each day. Up to 80 m3/day of leachate are generated, which have to be treated to very high standards. The treatment system is made up of a SBR with final polishing through a reed bed planted with Phragmites. The SBR is highly efficient for ammoniacal nitrogen removal from over 1200 mg/l to less than 1.0 mg/l. COD values are reduced by 60% from raw leachate values of over 2000 mg/l (Robinson et al., 2005).
MATERIALS AND METHODS
This chapter deals with the methodology adopted and is comprised of the following phases:
- Sample preservation
- Wastewater characterization
- Wastewater from SMTP
- Biological treatment of landfill leachate using a SBR
- Co-treatment of landfill leachate with wastewater from SMTP
- Results and analysis
Sampling is done to represent a certain population, in this case wastewater, on which tests are performed and the results symbolize the wastewater characteristics. This can be achieved by two methods: composite sampling and grab sampling.
A composite sample consists of collecting samples at regular interval in time. This will be representative of the average wastewater characteristics.
A grab sample is based upon obtaining a distinct sample regardless to its flow or time of the day.
If the wastewater quality is not highly variable, the results obtained from grab sampling will tend to corroborate composite ones. Both methods are used and for this project the grab sampling technique was adopted.
- Sample Preservation
Soon after the samples were collected, they were tested and if not possible, they were preserved. The latter is crucial step as most of the wastewater constituents have to be kept as are in their original state. They were incubated at 4° C and when necessary pH control was done by adding sulphuric acid. Subsequently, this will stop all the biological activities.
- Wastewater Characterization
The next step after sampling is characterization, i.e. determining the level of constituents present in the wastewater. As a fact of that, the treatment method is selected and applied to the polluting material. Each time, when new samples were obtained, they were characterized in compliance with Standard Methods of Testing.
For the project, characterization has to be done for these materials;
The leachates were delivered at the UOM Public Health Laboratory, on the 23rd October 2007 and 9th January 2008, and were characterized for the main polluting parameters. Then the sample was preserved till the treatment starts.
- Domestic Wastewater
The domestic wastewater was collected at SMTP. The sample was collected from the primary clarifier after degriting has been done on the following dates: 26th February and 3rd March 27, 2008. The samples were immediately characterized and then used.
For nitrification to take place there should be microorganisms feeding on the organic matter, but leachate does not contain any. Therefore, the returned sludge from SMTP was collected and brought to the UOM Public Health Laboratory. The sludge was allowed to settle and the supernatant was discarded, the residual left was used for testing. As a result of that the sludge concentration was increased and smaller amount is required for biological treatment. A TSS was carried out and the value obtained was used for calculations. The sludge was also studied under the microscope determining the microorganisms present and their conditions.
- Biological Treatment of Landfill Leachate using a SBR
The first option for treating leachate was the biological treatment by making use of a SBR. It was made up of the following phases: fill, react, settle and decant. The reactor consisted of sludge, water and leachate with varying composition. Their volumes were calculated such that the ammonia nitrogen concentration is about 50 mg/L in the reactor. The latter was aerated for a period of 24 hours. The main polluting parameters were monitored and accentuating upon the level of ammonia nitrogen and nitrate nitrogen. The system was run for a number of cycles and then denitrification phase was operated.
- Experimental Procedure
- A reactor of capacity 20 L was considered with an MLSS concentration of 4000 mg/l. The dissolved oxygen concentration had to be greater than 2 mg/l and this was achieved by the means of air diffusers. The diffusers provided the mixing within the reactor.
- Immediately after the setting out of the reactor, a grab sample was collected and was tested. These values were set as baseline.
- After 24 hours of aeration, another sample was collected from the reactor and tests were performed. The critical parameter i.e. ammonia nitrogen was observed and if, the value is not within the discharge limits then it aerated till the expected result is obtained.
- The biomass required nutrients which provided in the form of Potassium Hydrogen Phosphate.
- In order for the treatment to take place, we had to cater for alkalinity and this was achieved by the addition of concentrated sodium hydroxide.
- Thus the nitrification process was being monitored until no further treatment.
- A total of 3 sequential batch reactors were operated.
- After the operation of the third reactor, the denitrification phase was initiated.
- All the air diffusers were switched off and acetic acid was added to the reactor.
- The dissolved oxygen concentration was monitored till it reached the zero value and the nitrate nitrogen concentration was measured.
- Co-treatment of Landfill Leachate with Wastewater from SMTP
The other alternative is a combined method, treating domestic wastewater and leachate together. The treatment is biological in nature using a SBR with phases; fill, react, settle and decant. The treating system consisted of aerating the SBR, composed of sludge, domestic wastewater and leachate, for a period of 24 hours. The volume of leachate was gradually increased until no further treatment was observed. The main parameters were monitored, laying emphasis on the nitrification process. The values were recorded and analyzed.
- Small reactors of capacity 5 L each were considered with an MLSS concentration of 1500 mg/l.
- The first SBR was made up of 100% DWW and sludge only, the second one 95% DWW, 5% leachate and sludge, the third one 90% DWW, 10% leachate and sludge and so on. An example is being shown below.
- The dissolved oxygen concentration was kept greater than 2 mg/l by the use of air diffusers which also provided the mixing within the reactor.
- Immediately after the setting out of the reactor, a grab sample was collected and was tested. These values were set as baseline.
- After 24 hours of aeration, another sample was collected from the reactor and tests were performed.
- The volume of leachate was increased until the treatment stopped.
- Sometimes the phosphate concentration was too low and a phosphate had to be provided as potassium hydrogen phosphate.
- The experiment was repeated but with a reduced time of aeration.
All the tests were carried out at the University of Mauritius Public Health Laboratory. The tests were performed in compliance with Standard Methods of Testing for Water and Wastewater. Several tests were carried out such as pH, DO, COD, BOD, Alkalinity, TSS, Chloride and colour removal. The Hach 2000 Spectrometer was used for testing the following parameters: Ammonia Nitrogen, Nitrate Nitrogen, Phosphate and Sulfate.
- Results and Analysis
After obtaining the results, the values were verified and any discrepancy in them meant that corrective measures should be applied, for e.g. pH control, for the proper functioning of the reactor. The SBR was monitored on a day-to-day basis until treatment was brought to an end.
Analysis was done in order to determine the efficiency of the treatment methods. Ammonia toxicity was ascertained together with the percentage at which co-treatment can be practiced for the local context.
RESULTS AND DISCUSSIONS
This chapter has summarized all the results obtained along with some associated comments and consists of the following parts:
- The leachate obtained from the MCSLS was characterized and compared with the discharge limits.
- The microbiological characteristics of the sludge from St Martin wastewater treatment plant were assessed and their suitability for use was determined.
- The results of the biological treatment using a sequencing batch reactor were summarized with associated comments.
- The percentage at which combined treatment can be practiced was assessed as well as the level of ammonia toxicity.
5.2. Leachate Characterization
The raw leachate from MCSLS was characterized and the following observations were made:
- The ammoniacal nitrogen concentration of the leachate was very high, 1800 mg/L. Leachate do normally have a high ammonia nitrogen concentration and it remains approximately the same throughout the landfill life. In comparison with the discharge limits, for land/surface water and wastewater system, the permissible limit is highly exceeded and treatment is needed in order to prevent pollution.
- The chloride content of the leachate was high, 1172 mg/L and gave the leachate a dark brownish colour. The chloride concentration highly exceeds the discharge limits and a reduction in this parameter is quite costly. Biological treatment will not remove the chloride ions and more advanced wastewater treatment is needed such as reverse osmosis/ membrane filtration.
- The leachate had a low Sulphate concentration that was not detected by the colorimetric method, 0 mg/L. The other value was neglected since the sample was highly coloured and this gave an erroneous value. The value is well below the permissible limits for discharge and thus, no treatment is required for this particular parameter no treatment is required.
- The raw leachate had a COD concentration of nearly 5000 mg/L which a very high value and well above the discharge limits. The parameter must be treated to attain the permissible limits and biological treatment is suitable in such a case.
- pH values of 8 and 8.4 were measured during the characterization process. The values are within the permissible range and therefore no pH adjustment is to be made.
- The leachate had a low nitrate nitrogen concentration, 8 mg/L. The other value was neglected since the sample was highly coloured and this gave an erroneous value. The value is well below the permissible limits for discharge and thus, no treatment is required for this particular parameter no treatment is required.
It can be noted that all the tests were not performed on the second sample due to unavailability of some materials at that time. The first sample was taken during a rainy period, hence resulting in a lower value than the second sample.
5.3. Sludge Characterization
Activated sludge is made up of a mixed culture of microorganisms that metabolize and transform organic and inorganic substances into environmentally acceptable forms. The typical microbiology of activated sludge consists of approximately 95% bacteria and 5% higher organisms (protozoa, rotifers, and higher forms of invertebrates) (Wisconsin DNR, 2006).
The sludge consists of two main types of bacteria:
Heterotrophic Bacteria: They are the predominant bacteria in activated sludge as they are present in large numbers. They use organic carbon for cell growth. They participate mainly in aerobic oxidation of organic matter.
Autotrophic Bacteria: They are microorganisms that derive carbon cell from carbon dioxide and other inorganic materials. Nitrifying bacteria, Nitrosomonas and Nitrobacter, are autotrophic in nature.
The activated sludge from St Martin Treatment Plant was analyzed:
- To determine the various types of microorganisms coexisting
- To assess that they are sufficiently large in numbers to feed on the organic matter
- To check whether common sludge- related problems have arisen
Some of the microorganisms were identified such as rotifers, Trichocerca and Proales which were present in great numbers. Protozoans were identified which were in form of Opercularia SP and Vorticella Convallaria.
Filamentous microorganisms were found in the sludge which gave rise to poor settleability and therefore as a countermeasure the sludge was not stored for more than 1 week.
5.4. Biological treatment using a sequencing batch reactor
18.104.22.168. Variation in pH
The optimum pH range for the nitrifying bacteria is 7.2-8. Values less 5.5 and above 9 are critical and the process will be prone to failure. During the nitrification process, H+ ions are produced and the latter will lower the pH. This is shown by the following equation.
Therefore a buffer is needed in order to sustain the treatment process. Lime, sodium hydroxide and phosphate buffer were used from time to time to remediate the situation. The buffer solution provided, had a concentration so that it does not alter significantly the reactor volume.
The above graph clearly shows that a suitable environment for nitrification was given as the values lie within the optimum range.
22.214.171.124. Variation in Alkalinity
According to literature, for the nitrification process 7.14 g alkalinity (as calcium carbonate) is consumed per g of ammonia nitrogen oxidized. For every cycle, the ammonia loading was different and the alkalinity to be provided was calculated accordingly.
The alkalinity was increased by the use of the following buffers:
Phosphate buffer was used as it increased the phosphate concentration and subsequently nutrients were available to the microorganisms.
Lime was also used as a buffer so as to provide the autotrophic bacteria with inorganic sources of carbon from which they can derive their energies.
The graph shows the amount of alkalinity consumed over each cycle. When compared to the amount of ammonia nitrogen oxidized, the value for cycle 1 confirms the literature.
Theoretical alkalinity consumed = 7.14 ´ 35.1 = 250.6 mg/L
From graph alkalinity consumed = 247.5 mg/l
126.96.36.199. Variation in MLSS Concentration
The MLSS is the amount of biomass present in the reactor. It is important to determine the concentration of the microorganisms as it is closely related to the rate of nitrification A first reactor was set at a concentration of 5000 mg/L but due to poor settling characteristics, solids were obtained in the effluent. The food to microorganisms' ratio was low and therefore a new MLSS concentration was chosen.
The initial MLSS concentration in the reactor was set at 4000 mg/L. This value kept decreasing over each cycle. This is mainly because the sludge dried over the surface of the reactor after the liquid was lost due to evaporation.
After a sludge age greater than 8 days, the nitrifying bacteria become sufficiently large in numbers to exert an oxygen demand and nitrification can take place.
Nitrification is a microbial process, involving two distinct genera of microorganisms, Nitrosomonas and Nitrobacter. These autotrophic microorganisms build organic molecules using energy obtained from inorganic sources; in this case ammonia and nitrite are oxidized sequentially oxidized to nitrite and nitrate.
The overall nitrification reaction is given by the equation below:
At a concentration of 50 mg/L, ammonia nitrogen will start to adversely affect the biological process. It was a prerequisite that this value is not exceeded in the influent. The ammonia nitrogen was oxidized to nitrate nitrogen and it is dependent on several factors such as pH, DO and MLSS concentration.
From this experiment, a clarified effluent was produced with very low ammonia nitrogen concentration. The average percentage removal of ammonia is 85.3 % with a peak of 96.5 %. Yet, some of the values have not reached the permissible limit for discharge.
The graph shows variation in the nitrate nitrogen concentration. The latter had a value of 28 mg/L which gradually increased to 324 mg/L for the fifth cycle. Nitrification was taking place but at reduced rate. At the start of the fourth cycle, the colorimetric method for determining nitrate nitrogen concentration became inappropriate due to increase in sample colour. Thus, it resulted in erroneous values.
The COD level was reduced at the end of each cycle, showing that organic and inorganic compounds were consumed by the microorganisms. But this reduction was not significant as the amount of biodegradable fraction was few. It is believed that the leachate consists of approximately 50 % of hard COD based on previous works. These inert fractions will not exert an oxygen demand. The result may not comply with discharge limit for COD but its disposal may be considered to be safe.
Nitrification is usually accompanied by the denitrification process. Wastewater with a high nitrate concentration cannot be discharged in the environment; otherwise it may result in groundwater contamination. Denitrification is a biological process engaging facultative heterotrophic microorganisms.
The nitrification phase was stopped and the denitrification was started during the sixth cycle. The nitrate nitrogen concentration was 306 mg/L. For denitrification to take place, the following criteria were needed:
- Anoxic phase was created by aeration cut-off
- 3.7 mg/L of COD required per mg of nitrate nitrogen reduced
The first criterion was not satisfied as the DO concentration never reached the value of less 0.5 mg/L even after 5 hours of monitoring. Carbon source was provided by the addition of acetic acid but however this amount was insufficient. This might have cause the failure of the denitrification process.
- Combined Treatment of Landfill Leachate with Domestic Wastewater
5.5.1 Domestic Wastewater Characterization
The sample has a low concentration of the contaminants, mainly because the sample was collected during a rainy period. The sample is characterized by highly biodegradable COD, since during the operation for 100 % DWW by volume a reduction of 179- 33 mg/L was achieved.
The sample was slightly coloured and for nitrate nitrogen testing, a dilution factor of 100 was needed which may has induced an error in the reading.
Phosphate was present in the sample, not in high amounts but it was more than that was present in the leachate.
5.5.2. Variation in Dissolved Oxygen Concentration
For biological treatment to occur, a dissolved oxygen concentration of greater than 2 mg/L is necessary. Otherwise, the microorganisms will not survive as they are strict aerobes. For this labscale experiment, the DO was found to be highly greater than the limit at the start and at the end.
5.5.3. Variation in Dissolved Oxygen Concentration
For combined treatment, no phosphate buffer was needed and alkalinity was provided by lime and sodium hydroxide.
The graph shows the amount of alkalinity consumed over each cycle. When compared to the amount of ammonia nitrogen oxidized, the theoretical values of alkalinity consumed do not match the experimental ones.
It has been observed that the pH in the effluent was higher than that in the influent. The pH value during the reaction was slightly above 8. This pH is not favourable to the microorganisms for nitrification.
5.5.4. Variation in COD Level
The reduction in COD level is low with increasing leachate volume. Due to the adverse effects of ammonia concentration, most of the microorganisms have died resulting in fewer consumption of organic matter.
At 2.5 and 5 %, the COD values comply with the discharge limits and based on the effluent may be discharged in the environment for this parameter.
5.5.5. Variation in Ammonia and Nitrate Nitrogen Removal
Figure 5.10: Variation in Ammonia nitrogen and Nitrate Nitrogen concentration for co treatment
The domestic wastewater contained a low concentration of ammonia nitrogen and was rapidly treated to meet the discharge limits for land and surface waters. As the percentage leachate was increased, treatment became more and more difficult as the microorganisms were subjected to increasing ammonia toxicity.
At 17.5 % of leachate by volume, the nitrification process was inhibited and the ammonia toxicity was found to be approximately 160 mg/L.
After 16 hours of aeration, the maximum ammonia nitrogen removal was achieved at 2.5 % of leachate by volume.
6.1. Designing a sequencing batch reactor
The design is based upon experimental values obtained and the following assumptions;
- The maximum ammoniacal nitrogen concentration is 1800 mg/L in the raw leachate.
- For the year 2007, a total volume of 100 858 m3 was discarded and thus the daily leachate volume approximately equals 330 m3.
- An ammonia nitrogen concentration of 50 mg/L will adversely affect the microorganisms.
- A peaking factor was applied to the amount of leachate generated as it is highly dependent on the rainfall amount.
The following sequence was set for the reactor:
Influent ammonia from raw leachate: 1800 mg/L
Dilution factor: 1800/50 = 36
Daily leachate volume = 330 m3
Peaking factor = 2.5
Design volume = 2.5 ´ 330 = 825 m3
Volume of leachate that can be treated for 20 hours of reaction time
= (825/ 24) ´ 20 = 687.5 m3
Leachate volume to be stored = 825-687.5 = 137.5 m3
Reactor volume needed = volume of leachate ´ dilution factor
= 687.5 ´ 36
= 24750 m3
Tank volume » 25000 m3
Number of tanks to be used = 5
Volume of 1 tank = 25000/5
= 5000 m3
A dosing station is required in order to provide nutrients, alkaline buffer and carbon source for denitrification.
Nitrification kinetics (Eckenfelder,2000)
The nitrogen to be oxidized can be computed from
Nox = TKN - (NH3-N)e - (0.08 aH . Sr) - SON
Nox = Oxidized nitrogen
(NH3-N)e = Ammonia nitrogen concentration in the effluent
= 1 mg/L (assumed value)
SON = non degradable organic fraction
aH = Sludge mean yield coefficient
Sr = soluble substrate removed
= 65.2 mg/L (based on experimental results)
TKN = Total Kjeldahl Nitrogen
From literature, ammonia nitrogen forms 70 % of the TKN. The influent ammonia nitrogen concentration is 50 mg/L, given a TKN value of 62.5 mg/L.
Nox = TKN - (NH3-N)e - (0.08 aH . Sr) - SON
= 62.5 - 1- (0.08´0.5´65.4)-1
= 57.9 mg/l
Oxygen required = 4.33 Nox
= 4.33 ´57.9
= 250.71 mg/L
Alkalinity required = 7.15 Nox
= 7.15 ´57.9
= 414 mg/L
0.251 kg/ m3 are needed as aeration requirement and0.414 kg/ m3 alkalinity provision. In order to maximize on efficiency of the diffuser system, the depth of the tank has been reduced as well as for easier maintenance purposes.
There is a restriction for sizing the radius of the tank as inadequate zones of mixing have to be minimized.
6.2. Design of a tank for combined treatment
From the experimental results, the optimum leachate percentage was found to be at 2.5% for 16 hours of reaction time.
A daily volume leachate of 330 m3 is generated and the daily flow at St Martin wastewater treatment plant approximates 60 000 m3. The flow is sufficient enough to accommodate the landfill leachate.
6.3. Tank sizing
Peaking factor = 2.5
Design flow = 330 ´ 2.5
= 825 m3
Tank volume = 825 m3
The tank has dimensions 11 m ´ 15 m with a depth of 5 m. The tank will accommodate a leachate volume of 825 m3. The leachate is mixed and aerated where it will enter the existing treatment system at a very low rate. Hence, the leachate will not disturb the system and treatment of leachate is carried out.
CONCLUSIONS AND RECOMMENDATIONS
The aim of the project was to the study of nitrification of landfill leachate and choosing a suitable treating system to eliminate key contaminants. Two treatment options were identified both being biological in nature namely, operating a sequencing batch reactor and the combined treatment with domestic wastewater. Both treatment options are usually described as being cost effective and easier to operate. This part of the chapter deals with some notifications made throughout the project and some recommendations that could help to improve the method of treatment.
7.1. Comparison between SBR and Co treatment
As it can be seen from the above table, the operation of a SBR implies greater costs than co treatment. The daily volume of leachate that can be treated is less for the SBR. There is an extensive land use for the construction the sequencing batch reactor.
The amount of input such as buffer solution, nutrients and aeration for co treatment is less. The SBR needs to work in conjunction with other treating systems in order to meet the permissible limits. Hence, co treatment turns out to be simple and cost-effective process for leachate treatment.
- After the operation of the SBR, usually a secondary treatment is applied to polish the treated wastewater. It can be noted that the effluent has not met the discharge limit for land/ surface waters. Therefore, a secondary treatment option is needed and can be in the form of addition of powdered activated carbon, reed beds or ozonation.
- There was a significant drop in pH level during the whole treatment process. As illustrated in the design part, the amount of alkalinity required is quite significant and will incur high costs. The operation of a denitrification phase just after the nitrification one is beneficial as the amount of alkalinity required can be reduced by two.
- For the combined treatment, uniform mixing is required before the wastewater enters the aeration unit.