Innovative wetland system gains ground
A technology designed for wetland treatment of landfill leachate can help optimise other wetland sites, say Craig Fannin, associate director, and Paul Danes, senior consultant, with AMEC's UK Earth & Environmental business
Although only applied to landfill effluents at present and being considered by a third leading UK national landfill operator as a potential solution to the management of leachate at its closed sites, the system can also be utilised for other effluents requiring sustainable treatment. The versatility of the system for various potential pollutants is illustrated by the first two HaloTech systems that have been commissioned on closed landfill sites during 2009 and 2010 for the primary treatment of approximately 50m3/day of mixed effluents.
The first on Viridor's Westbury site in Wiltshire, from a municipal solid waste (MSW) leachate has been optimised for ammonia and BOD reduction under aerobic and anoxic conditions. The second at a Lafarge Cement site at Broadness in Kent has been optimised for copper removal under sulphate reducing conditions from an organic-free cement kiln dust (CKD) leachate. The process works as a passive system following dosing and can be operated throughout the year with minimal management input. In fact the sulphate reducing system is primarily designed to operate over the winter period, with minimal throughput during drier seasons.
Treatment wetlands are increasingly being utilised as sustainable low impact solutions to managing effluents and wastewaters from domestic sewage through to mining and other industrial effluents. Treatment wetlands are still perceived as an effective low technology solution to pollution control for low strength waters or for tertiary treatment of more difficult effluents, particularly in remote locations or where the aesthetic impact must be considered.
There is a technology gap between conventional wetland systems and the more intensive treatment processes for concentrated effluents using technologies such as aerobic and anaerobic biological reactors or membrane systems. These technologies have advantages associated with high throughput, performance and limited footprint requirements whereas the negligible intervention and power consumption benefits of wetland systems have come at the cost of reduced influent concentrations.
Primary wetland systems employed to date have focused predominantly on aerobic, denitrifying or evaporation processes for limited BOD loads associated with agriculture or remote sewage effluents and closed landfill sites. Alternatively sulphate reducing systems are employed where the treatment target cannot be chemically or biologically degraded to gaseous products such as for mine effluents.
HaloTech was originally developed as an alternative to biological reactors for the removal of BOD and ammonia where solution salinity was producing additional difficulties for specialised landfill leachates. However, the treatment processes optimise the same treatment mechanisms as other wetland systems, filling the gap between the performance capability of traditional wetlands and more intensive processes.
The system is analogous to a fixed bed bioreactor, where the plants act as sub-surface aerators and natural bacteria are cultured as the primary treatment mechanism. The treatment bacteria are protected by concentration gradients across a mineral mounted surface biofilm which shields the bacterial community from the higher concentration levels that would be considered toxic or inhibitory to a planktonic biomass. The majority of constructed wetlands are employed in the treatment of nitrogen rich wastewaters, with the majority of the nitrogen in the reduced form as ammonium. The process of nitrification within biological treatment systems treating nitrogenous wastewaters is a well known and documented process along with the process of denitrification.
The understanding of the biogeochemical processes within wetland system is ever increasing, allowing the further advancement of wetland systems and increasing the variety of applications accessible by wetland systems.
HaloTech is operated as a vertical-flow constructed wetland using a soil-compost substrate and salt-tolerant marsh plants, instead of the more traditional Phragmites reeds. The biologically mediated treatment occurs through a series of soil-microbial reactions in a symbiotic relationship between the plants and microbial communities, with each bacterial community occupying a specific ecological niche.
It is the controlled management of these niches during seepage through the treatment substrate that allows organic carbon removal in aerobic or anaerobic conditions; nitrification under aerobic conditions; complete nitrogen removal under anoxic and anaerobic conditions; sulphate reduction and associated priority metal removal. The managed control of the aerobic and anaerobic conditions within the wetland allows the microbially mediated anaerobic ammonium oxidation mechanism, to be optimised resulting in over 90% nitrogen reduction from influent ammoniacal-N concentrations between 700 and 1,100mg/l.
Anammox utilises the nitrate and nitrite formed from the partial nitrification of ammonia under oxygen deficient conditions.