Joining forces against ammonia
Ammonia pollution into the environment is causing increasing concern because of its threat to sensitive habitats. Stan Cheshire looks at ways WwTWs can play a key role in minimising the impact.
An effective approach to wastewater can help bring the total ammonia and nitrogen levels below critical load thresholds.
However, conventional solutions to ammonia can pose a challenge for wastewater treatment. They can require a large footprint because of the high volumes involved or through the use of horizontal sand beds. And increasing the levels of sludge concentration can be a problematic alternative.
More novel, space-saving solutions based in an up-flow principle are being sought to achieve regulatory requirements at a reasonable cost.
The Environment Agency (EA) and Defra believe the way forward is to integrate the activities of wastewater treatment with other contributory factors and considerations - particularly agriculture, land use planning, biodiversity, tourism, leisure and recreation, flooding and health.
Ammonia pollution is a key consideration in all of the above. Firstly it is a cause - for example through agriculture practices.
Secondly, it is a disseminator - stormwater flooding and run-off can instigate pollution incidents. Finally, the pollution may affect biodiversity and river and coastal tourism and leisure.
Joining up the reduction of agricultural ammonia generation and diffusion problems with land use planning is a major start. More than 80% of the ammonia produced in the UK is agriculturally derived.
Co-ordination of, say, planning licences for new pig or poultry units with the strategy for sensitive habitats downstream in the river basin is essential to ensure that the effects of eutrophication and excess nitrification are minimised.
The European Commission's (EC) Habitats Directive (1992) was a major step forward in conservation, enabling several subsequent initiatives to be set up such as defining specific habitats, protected and conserved areas, including river basins and coastal areas. Further directives, including the shellfish and bird directives, have and will be consolidating these initiatives.
They are being brought together under the Water Framework Directive, which was adopted in 2000, and is scheduled to be fully implemented under national law in member states by 2015.
WwTW nitrogen reduction in discharge waters is based on the production of relatively inert, dissolved gaseous nitrogen by harnessing the bacteria in urea and providing the right conditions for the right species of bacteria to flourish.
Conventionally, this takes place in basins that require significant tank volumes. Many existing treatment plants are too small to accommodate the volume required, and attempts to achieve this through increasing sludge concentrations and sludge residence can lead to other problems.
In addition, settling tanks for removing the bacteria may well be required.
Some older treatment plants have horizontal sand bed filters, which allow the bacteria to settle on the solid surfaces of the sand. These achieve much higher concentrations than in open tanks and provide a large surface area for action.
However, they also occupy much space, and need equipment for dispersing the raw fluids, which requires maintenance. Additionally, they have a fixed useful life and possible major costs because the filter beds get clogged up and have to be dug out and disposed of or even abandoned.
The alternative is to install upflow vertical tank biological sand filters that require much less space - typically one tenth of the basin alternative.
By continuously cleaning the sand, the effectiveness of the filter system is maintained and the sediments can be washed out for disposal. The continuous sand cleaning also maintains the health of the bacterial population.
As the water flows up through the sand and through an undisturbed upper volume in water to cascade over a weir, the bacteria remain in the sand. This saves the requirement for additional settling tanks, to remove bacteria from the water.
The Dynasand filter uses the upflow principle and contaminated sand is also circulated constantly via an airlift pump. Thus, for a small power input, cleaning of both the water and the sand is continuous and the filter need never be shut down.
This process has now been further developed to provide highly effective treatment for the removal of ammonia from the water, by creating favourable conditions for the treatment bacteria to flourish within the sand filter.
As the biomass and sand are continuously moved through the filter and the most favourable conditions for growth, the filters are also very tolerant of ammonia surges.
To ensure that the autotrophic nitrifying bacteria have sufficient oxygen and carbon dioxide to convert ammonia to nitrates, aeration hoods fed by process air are sited within the upper layer of sand.
They provide a mixing action, which allows sufficient aeration. And the resultant nitrates are dissolved in the water. This version is termed the Dynaoxy.
To provide denitrification as part of the treatment package, a version called the Dynadeni filter can be configured without the aeration hoods and a deeper filter bed. This enables more anoxic conditions to be created. And it allows the denitrification bacteria to flourish using a controlled dose of methanol as a carbon source. And the nitrogen is released as gaseous nitrogen to the atmosphere.
For a complete package to eliminate ammonia and nitrates, the two processes can be arranged side by side, with the denitrification filter downstream and gravity fed from the nitrification / ammonia removal filter.
In a typical installation, Wessex Water installed three Dynaoxy nitrification filters at Merriott WwTW to polish the treated water prior to release to the local watercourse. The Dynaoxy provides nitrification, filtration and aeration in one system, so is very economical of space, with a small footprint.
The three 9.123m-high filter units with associated pipework, compressors, blowers and stairway access were installed on a 12.5 m by 6.6m concrete slab.
"Two years on, the filters are continuing to perform very satisfactorily and exceeding the specified standards," says Steve Tomlin, technical manager at Wessex Water. "It is achieving less than 1.0mg/l of ammoniacal nitrogen on average, and the BOD and solids levels are also very good."
Stan Cheshire is UK sales manager at Hydro International's water and wastewater Division.
T: 0800 269371.