Landfill Leachate Treatment Simple as One, Two, Three Leachate treatment facility at Sita’s Arden Quarry landfill site The treatment of landfill leachate needs to be efficient, reliable and as simple as possible. Unfortunately many systems are over engineered or do not take into account the biology of the process. In essence, the system must look after the bacteria; otherwise the bacteria will not treat the effluent. In this article we will review some basic facts regarding innovative technology in a leachate treatment system, which has been running for the last 18 months with 100% compliance with the discharge consent. The Concept. Bacteria, protozoa, algae and multi cellular organisms will develop in the treatment system. It is the activity of these organisms that is responsible for the treatment of the effluent. The treatment system is only a life support mechanism to enable the organisms to perform at their best. The principal group is the bacteria, and they have some basic requirements that must be satisfied. Bacteria measure around 0.5 to 5 microns in size and there are millions of different species of bacteria which can assist with treating leachate. Some bacteria prefer high BOD, or ammonia, and require oxygen for growth. Other bacteria are incapacitated by oxygen, whereas some can adapt to aerobic and anaerobic conditions and change their metabolic pathways depending on circumstances. Bacteria can also change to become adapted to an environment and exotic “food sources” such as PCBs. Give bacteria the right conditions and they can perform a wide range of treatment techniques such as assimilation, decomposition and de-toxification of the effluent, all techniques should be encouraged in a stable leachate treatment system Bacteria prefer to live in colonies, which is called floc. By providing numerous different environments in the treatment system it is possible to ensure that there is a wide species diversity. The advantage of this is a stable biomass population, which will be capable of processing leachate successfully under a wide range of conditions. How then do we achieve this task? Bacteria stability. The bacteria floc is like a small fragile ball of cotton, the bacteria act like a colony that supports each other. The larger the bacteria floc the more stable the colony and the better it will perform. The colony requires food and usually oxygen, the food comes from the leachate and the oxygen from the aeration system. The aeration system must provide sufficient oxygen to ensure that the levels do not fall below 2mg/l. However best performance is achieved by the bacteria if the oxygen levels are maintained above 5mg/l. The higher oxygen level will require more energy, however the improved bacterial performance is justified by the higher dissolved oxygen concentration. Techniques used to provide oxygen include, surface aerators, venturi injectors and air diffusers. Surface aerators mechanically aerate the water; they will cause some damage to the bacterial floc and they use a great deal of kinetic energy and are generally not as efficient as other means of aeration. Venturi injectors have a high oxygen transfer coefficient, but during the passage of the bacteria through the injector, the bacteria are exposed to extreme shear forces and pressure gradients, which breaks up the bacterial floc. If the injectors are using air and not pure oxygen, then nitrogen gas in addition to the oxygen will be dissolved into the water. The nitrogen will come back out of solution in the aeration tank, just like taking the top off a lemonade bottle. Small bubbles of nitrogen will stick to the bacteria, and develop inside the bacteria. The nitrogen bubbles give the bacteria buoyancy, which prevents them from settling. Degassing of nitrogen inside the bacteria effectively gives the bacteria the "bends", exactly the same condition divers experience from rapid decompression. The nitrogen will increase the internal pressure within the bacteria and in some cases they can actually explode. These conditions are hardly conducive to the development of a stable bacterial biomass. Bacteria suffering from the “bends” are often characterised by poor settlement characteristics and carry over into the final effluent. Aeration, especially fine bubble diffused aeration, provides a high level of oxygen transfer, and is very gentle with the bacteria. Large bacterial flocs up to 5mm in diameter will develop and settlement velocity and clarification of the leachate is excellent. There has been a move towards covering lagoons and tanks in order to prevent emission of odours. However if the systems are properly monitored with dissolved oxygen probes, and oxygen levels maintained above 2mg/l, then there will be no release of odours. By using open tanks or lagoons further assistance in the treatment of leachate is provided by photosynthetic algae. The activity of algae has been completely ignored in many effluent treatment systems. They are excellent scavengers for ammonia, phenols, list 1 substances and heavy metals. Algal based effluent treatment systems are in the early stages of development, however, by not covering the tanks, it gives the opportunity for algae to develop. As a by-product, algae produce oxygen which helps support the aerobic bacteria, yet another example why species diversity increases system stability. Even when the aeration systems are turned off, the oxygen levels can actually rise in the surface water of the treatment tanks due to algal activity. Environment niches The bacteria need to live in colonies and there needs to be a high species diversity of organisms for best performance by the system. If we take a number of tanks, all connected in series in which the leachate flows from one tank to the next, the treatment of leachate can be divided into the following stages:- Step 1 Raw methanogenic leachate from the landfill site will have high levels of oxygen demand (COD and to a lesser extent BOD) and ammonia. In an aerated aerobic tank, heterotrophic bacteria, which utilise organic carbon as a food source, will develop. Given sufficient oxygen and time they will break down much of the COD and all of the BOD to carbon dioxide and water. They also utilise ammonia as a source of nitrogen for protein synthesis. Heavy metals and persistent organics, including some pesticide residues, will also be taken in by the bacteria and incorporated into their cell biomass. When the bacteria have exhausted the food supply in the leachate, i.e. the readily oxidisable content has been consumed, then they will become cannibalistic. They will feed on any available oxidisable carbon source and this includes any dead bacteria in the tank. The so- called endogenous respiration phase prevents the build up of sludge and some treatment systems have been operated successfully since 1992, without the need for sludge removal. Step 2 Some bacteria oxidise ammonia to nitrate. They are known as the nitrifyers and are autotrophic bacteria, which utilise an inorganic carbon source for cell synthesis. They are renowned for being temperamental, have a very slow growth rate and have great difficulty competing with the heterotrophic, or COD reducing bacteria. They are essential for the oxidation of ammonia, however, and are a vital component of the leachate treatment system. They need to be protected. The autotrophic bacteria like to stick to a surface, so we help them by including a floating plastic bacterial support media in the aeration tank. The plastic biofiltration media increase the surface area within the tank for colonisation by bacteria to several thousand square metres. By operating at least two treatment tanks it is possible to crudely segregate the heterotrophic bacteria from the autotrophic bacteria. Both types of bacteria will be in each tank but they perform better in the tank that provides the conditions that they prefer. The net result of a multiple tank system is that the treatment system will be more stable, adaptable and will have a greater treatment efficiency by enabling a wider range of organisms to develop with greater species diversity. Step 3 Even with the introduction of biofiltration media in the tank, bacteria will be present in the final effluent and can be lost from the treatment system. This has two major disadvantages:- 1. The bacteria contribute to the suspended solids and may breach the consent conditions of a discharge to surface water. 2. The bacteria will result in a charge from the water company for suspended solids if the discharge is a trade effluent consent. It was mentioned previously that the autotrophic bacteria are slow growing and they are the workforce for the oxidation of ammonia. To allow them to be lost from the treatment system is undesirable and is a problem faced not only in leachate treatment but also sewage treatment by the water industry. The innovative solution at the Arden Quarry landfill site, operated by Sita, has been to filter the leachate, using a tertiary treatment step, directly from the aeration tank to remove the bacteria from the final effluent. The effluent is discharged to sewer after being held in a balancing tank and the suspended solids content is typically less than 1mg/l. The bacteria that are intercepted by the filter are recycled back to the aeration tanks every 6 hours to ensure that the bacteria are still viable. By the use of this tertiary treatment technique the essential nitrifying bacteria are retained in the treatment system. The leachate at Arden Quarry is typical of many landfill sites. The ammonium levels range from 500 to 1000mg/l, and the COD from 500 to 3000mg/l. The water volumes treated on a daily basis range from 50 to 200m3/day. Fig 1 shows the level of ammonium in the discharge from start-up of the treatment system, over a period of 1 year. After start-up it took approximately 4 weeks for the bacteria to achieve satisfactory results. The level of ammonium entering the system ranged from 500mg/l to 1000mg/l with the average around 800mg/l. The final effluent quality was extremely stable and never exceeded 3mg/l after the system was conditioned. Fig 1. Ammonium level in treated leachate Discharge ammonium levels for landfill leachate influent = 500 to 1000mg/l 25 20 ammonium mg/l 15 10 5 0 22/07/2002 05/08/2002 19/08/2002 02/09/2002 16/09/2002 30/09/2002 14/10/2002 28/10/2002 11/11/2002 25/11/2002 09/12/2002 23/12/2002 06/01/2003 20/01/2003 03/02/2003 17/02/2003 03/03/2003 17/03/2003 31/03/2003 COD The COD levels range from 500 to 3000mg/l with an average of approximately 2500mg/l. It is relatively easy for the bacteria to remove approximately 80% of the COD, however the remaining component becomes increasingly difficult for the bacteria to digest. Fig2 below shows that the bacteria were able to reduce the COD level by approximately 90%; the remaining fraction is classified as hard COD. It is possible to remove this hard COD by cracking the organic molecules using oxidation techniques followed by biological treatment, removal levels better than 98% can then be achieved. Fig 2. COD level in treated leachate COD discharged for landfill leachate influent = 500 to 3000mg/l 500 450 400 350 COD mg/l 300 250 200 150 100 50 0 22/07/02 05/08/02 19/08/02 02/09/02 16/09/02 30/09/02 14/10/02 28/10/02 11/11/02 25/11/02 09/12/02 23/12/02 06/01/03 20/01/03 03/02/03 17/02/03 03/03/03 17/03/03 31/03/03 Tertiary Treatment Effluent treatment for the final removal of suspended solids is not new. A variety of clarifiers and sand filtration systems are available. The inherent problem with sand filtration has been irreversible biofouling of the media by the bacteria that have been intercepted. Simple backflushing is not sufficient to remove the accumulated solids and even with air scouring and the addition of cleaning chemicals the performance of the sand filter deteriorates, sometimes after a matter of weeks. To protect the sand filter, good settlement is required and the target maximum solids concentration in the feed effluent is usually less than 50mg/l. At Arden Quarry the feed to the tertiary treatment system is taken directly from the tanks whilst they are being aerated. The solids loading can be up to 1500mg/l. This would be suicidal for conventional sand filtration systems. AFM filtration AFM (Active Filter Media) is the filtration media used at Arden Quarry. It is similar in appearance to sand and is used in a standard pressure sand filter. AFM is manufactured using recovered green and brown glass bottles, which have been processed to create a filtration media that is resistant to biofouling. Examination under the electron microscope reveals that AFM has a very smooth surface compared to sand and it is this smooth surface, which prevents bacteria sticking to the surface of the filter media. Simple backflushing is sufficient to restore the filter to full capacity even with a solids loading in the raw effluent feed some 30 times greater than would be acceptable in a conventional sand filter. In addition to being able to remove much smaller particles from water than sand, AFM also has an overall negative charge at its surface, which can attract heavy metals and persistent organic contaminants. Table 1 shows that the leachate at Arden Quarry has prescribed substances at very low concentrations at, or about, the limit of detection. The level of prescribed substances is at its highest in tank 3, where they have been concentrated by bioaccumulation during many months of treatment. Significantly the concentration is below the limit of detection in the final effluent. Table 1 Removal and Assimilation of Prescribed Substances Raw Tank 3 Final leachate effluent Tributyltin <0.0398 0.058 <0.0398 Tributyltin 0.069 <0.05 <0.05 Mercury 0.097 0.160 <0.09 Mercury <0.1 0.105 <0.1 Results in µg/l The AFM filter at Arden Quarry is only a small unit taking up six square metres of space. It will, however, process 250m3 of effluent per day. Larger leachate volumes can be processed by using multiple filters in parallel. Since July 2002 it has intercepted bacteria from the aeration tanks and has virtually eliminated the discharge of bacteria and other organisms as suspended solids. The bacteria have been salvaged and recycled back into the treatment systems. The system has proved to be very robust and has been compliant with the trade effluent consent and has removed prescribed substances to below the limit of detection. The discharge from Arden Quarry is authorised under a trade effluent consent issued by United Utilities under the Water Industry Act 1991. The receiving sewage treatment works is up to treatment capacity and non-compliance with the trade effluent consent could have serious consequences for both Sita and United Utilities. Brian Ellor, Trade Effluent Policy Manager for United Utilities, comments “The trade effluent discharge from Arden Quarry is received at a United Utilities sewage treatment works which discharges into sensitive and high-quality controlled waters. The leachate treatment system has been operational since July 2002 and we have experienced no problems so far. The successful use of a tertiary treatment technique, which can process final effluent directly from an extended aeration system, is highly innovative. We at United Utilities are looking at AFM for the removal of certain contaminants from some of our sewage treatment works final effluent. The fact that AFM is a recovered material is another environmental bonus for the product.” A series of demonstration projects are being undertaken with AFM as part of a detailed R&D programme. The research is supported by Life Environment and WRAP (Waste and Resources Action Programme). Andy Dawe, responsible for WRAP’s Glass Programme says, “The results which have been achieved using AFM in landfill leachate treatment, sewage treatment, swimming pools, industrial process water treatment and even drinking water filtration have been universally successful. This high quality filter media has the potential to radically reduce the incidence of effluent non-compliance. This is an excellent example of where recycling not only reduces landfill, but also helps to ensure that the environmental impact of the landfill is itself reduced.