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 Ð¨ 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.
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