MBR plant first for leachate treatment
Cleanaway has commissioned a membrane bioreactor plant for leachate treatment on its landfill site in Essex that claims to offer a number of advanatges over conventional solutions
The plant, which is due to go live later this year, will provide a 200 m3/day treatment capacity at Cleanway's Ockendon landfill site in Essex, enabling the company to expand its site while maintaining its discharge limits for ammonia.
Extensive site tests were carried out on the leachate at Ockendon. Successful completion of the treatability trials demonstrated the suitability of the MBR process under a variety of operating conditions. Following this, Cleanaway placed an order with Wehrle Environmental for the MBR plant.
Despite its novelty in the UK, MBR is a well-proven process in the European landfill sector. While similar to conventional leachate treatment, MBR claims to offer a number of advantages over the most commonly used processes such as sequencing batch reactor (SBR). These result from the method used to separate treated leachate from the activated sludge - the water/biomass mix that treats the leachate.
Conventional processes use gravitational forces to achieve this separation. While this generally works well, a high concentration of biomass in the sludge can lead to increased suspended solids being discharged with the treated leachate. MBRs, in contrast, employ membrane technology that allows a much higher concentration of biomass to be used while ensuring that the final effluent is free of suspended solids.
This removes the need for large and expensive bioreactors, as higher sludge concentrations mean that a greater mass of biomass can be concentrated within a much smaller volume. With an MBR system, a ten-fold reduction in tank volume is achievable presenting a real benefit to customers should space be limited or the construction of large tanks prohibitively expensive.
Wastewater enters a reactor where it undergoes intensive aeration. The biomass uses the provided oxygen to degrade organic compounds and ammonium to carbon dioxide and nitrate respectively. Following the biological treatment step, the biomass is pumped to the ultrafiltration plant.
It is here that the separation of biomass and treated effluent takes place. Driven by pressure, only water and dissolved compounds can cross the membrane walls while the concentrated biomass returns to the biological step where it is used again in the treatment process. The treated leachate is completely free from suspended solids. In addition, the high biomass concentration and intensive aeration ensure stable quality of the permeate.
At Ockendon the reduced volumes of MBR tanks were a crucial factor in Cleanaway's decision. The leachate treatment plant will be constructed in close proximity to residential properties and planning restrictions placed on the height of structures mean that all tanks will need to be partially buried. As MBR tanks are compact due to high sludge concentration, the cost of excavation is considerably reduced.