Leading provider of water, waste and environmental solutions WRc looks at in-situ denitrification
Now the EC has set permissible levels for nitrates in drinking water, the hunt is on for a sustainable, cost-effective method of de-nitrification, preferably at source, which produces no waste.
Nitrates principally find their way into groundwater from diffuse sources related primarily to land use and agricultural management which is heavily dependant on nitrogen applications. There are many designated nitrate vulnerable areas in the UK, and measures have been introduced to reduce agricultural inputs of nitrogen. Even so, nitrate concentrations are continuing to rise and about 10% of UK public supply groundwater boreholes now exceed the maximum permissible concentration. In many cases, water can be blended to reduce the nitrate concentration, but this is not always a practical nor a long-term solution. The cost of treating for the removal of nitrate can be considerable.
WRc and Imperial College have been carrying out research and mathematical modelling since 1993 on harnessing naturally-occurring microbial denitrification activity in the saturated zone within aquifers. The team has now presented a feasibility study and economic assessment of in-situ denitrification treatment systems together with a design for a proposed denitrification facility. WRc is convinced this technique is technically feasible, economically viable and environmentally sustainable.
The system is at pilot plant stage, an industrial-scale version is due within two years. WRc intends to work with partners and is seeking suitable sandstone or chalk/limestone aquifer sites on which to install pilot treatment systems.
In-situ denitrification plants are used in the USA and Europe. The treatment facility usually comprises a ring of injection boreholes, 20-50m deep, depending on aquifer depth. A carbon source, WRc is using glucose, is injected into the boreholes. The naturally occurring microbial activity will reduce nitrates to nitrogen gas which is then released to the atmosphere. This naturally occurring phenomenon is enhanced and stimulated by the addition of the carbon energy source and produces no waste. The level of microbial activity can be controlled closely through the rate of glucose injection. Care must be taken not to introduce too much glucose as excessive bacterial growth can foul the system leading to reduced water supply. Too little glucose will not reduce nitrate concentrations.
The WRc study showed microbial activity increased within hours of introducing the carbon source, and nitrate concentrations in the water were reduced below the EC limit of 50mg NO3/l, from an initial value of 110mg NO3/l, within 10 days. The groundwater temperature was 12°C, with glucose and oxygen inputs kept at 40mg/l and 8mg/l respectively.
The optimum configuration of the facility is known as the ‘daisy wheel’. At abstraction rates of up to 1.5Ml/d, from a sandstone aquifer, eight carbon injection boreholes sited 10m from the central water abstraction borehole will be sufficient to treat the groundwater. However, if abstraction rates are increased up to 5Ml/d, the number of injection wells would be increased to 12. In a full-scale plant, the system control would be fully automated.
So far mathematical modelling by WRc and Imperial College has produced a transport model simulating solute movement under saturated conditions, and a new biochemical model describing the integration of solutes and biomass. These models are fundamental to the detailed engineering design and configuration of the facilities. Cost benefit analysis carried out by the team showed this method of treatment is a highly competitive alternative approach to nitrate treatment compared with the conventional surface-based engineered processes. WRc engineers compared the in-situ denitrification process with electro-dialysis, ion exchange and reverse osmosis. Overall, the new method could save an estimated 60% on CAPEX and OPEX costs. Specifically, there are no costs associated with the disposal of nitrate-rich waste, since the system produces no effluent discharge.
Because it is inexpensive to install and easy to operate intermittently, the new approach has particular advantages over more engineered systems for aquifers in which nitrate concentration only exceeds the limit on a seasonal basis. It also has a small footprint and is visually unobtrusive. WRc is looking to the start of AMP4 in five years time, and expects to see the potential of these plants fully recognised in the UK.
WRc has identified potential pilot sites at a greensand aquifer site in Hertfordshire and chalk site in Wiltshire. The company is looking for other potential sites as well as partners to sponsor the project. A number of utilities have already shown an interest, as has the Environment Agency (EA) and Drinking Water Inspectorate (DWI).
Daisy wheel denitrification facility
1) nutrient feed pipe to injection borehole
2) 10m radius around borehole one
3) site fence
4) two mixing tanks
5) pipe to take 10% of borehole one discharge for injection
6) pipe to take 90% of borehole one discharge for waste
8) borehole one
9) distribution tank / mechanism
10) borehole two
11) hot water cylinder
12) compensation drain
13) injection borehole
14) glucose delivery
15) glucose storage
16) Tarmac hard standing
Injection rate for each injection borehole is 0.0125 of the discharge.
Equipment and pipeline layout is indicative only, final design will depend on site conditions.