Treating sewage can be an unpleasant and costly business. Complex infrastructure is often energy intensive and the resulting sludge still has to be treated and disposed of in its own right.
In the wilds of Scotland, however, a new type of wind powered biological treatment works is currently treating sewage for 2,000 people and producing not a scrap of malodorous by-product.
'If you eliminate the sludge, you eliminate a large part of the cost of running a treatment works, ' says MWH project manager Ian Wynnes. 'You don't have to landfill it, put it on land or treat it.'
The system, built at Errol for North of Scotland Water (NOSWA) by contractor MWH and US technology firm LAS International, harnesses a biological process in which solids are fully degraded by balancing aerobic and anaerobic digestion.
Processes are enhanced and odours reduced by oxygenating the sewage in two large cells which present a large surface area, crucial for proper aerobic exchange. Physical mixers are used to aerate the sewage, and oxygen levels are topped up by injecting air.
At Errol, sewage is transferred to the works via the local pumping station and screened at the inlet structure.
Primary and secondary treatment takes place in cells, created by 1.3m deep excavations surrounded by a 3m high embankment of spoil sourced from the local landfill. Both cells are lined, and minimising visual intrusion was a major consideration, Wynnes adds.
In the primary cell, the solids are allowed to settle out in a digestive zone, with biological oxygen demand met with the aid of mixers driven by a pair of wind-powered sails. The mixer arms have been custom designed to keep dissolved oxygen levels at an optimum 2-4mg/litre.
Sensors ensure oxygen levels are constant. As a precaution , an electric motor is on standby for windless days so that mixing does not stop. However, Wynnes explains, 'if oxygen falls below 2mg/litre, we blow air into the bottom of lagoons, which creates a 'racetrack' flow', circulating the sewage and ensuring even replenishment of oxygen levels.
Diffused aeration is automatic, achieved using a low maintenance centrifugal fan blower.
Once primary treatment is complete, sewage is moved into the second cell for further mixing and aeration, which reduces nitrate levels and treats remaining faecal contamination. The resulting effluent is discharged to the Tay estuary.
By relying primarily on wind power, energy consumption at Errol has been kept staggeringly low. The system can run on energy equivalent of boiling four kettles of water - a power demand of 11kW/day. According to Wynnes, this is typically 65% to 85% lower than the energy consumption of a typical activated sludge process.
Further savings on running cost have been made by using stainless steel components in the dissolved oxygen system, removing the need for routine cleaning or maintenance. The system has been installed under a 20 year guarantee.
Errol's zero sludge biological sewage treatment process traces its arrival in the UK to 1998 when MWH, formed last year when consultant Montgomery Watson merged with US energy firm Harza, was approached by LAS.
LAS had been selling the technology in North America for the past 25 years but was having problems breaking into the UK wastewater treatment market.
Biological treatment systems had been tested abroad and found wanting.
'Waste stabilisation ponds had got bad press because some of the less efficient biological systems used created odour problems, ' says Wynnes. Resolving those problems was relatively straightforward, he adds:
not enough care had been taken to ensure high oxygen levels in the sewage treatment cells. As a result, too much anaerobic degradation was taking place, producing smelly methane gas.
Until last year, NOSWA was discharging sewage from Errol straight into the Tay estuary, and siezed on the idea of biological treatment after talking to LAS in 1998.
The water authority was at first impressed solely by the fact that no sludge disposal would be required, making for simpler operation, recalls Wynnes. But NOSWA quickly latched on to the cost savings offered. With no sludge there was no need for sludge clarifiers and sedimentation tanks. At the same time, access roads could be built to a lower grade as there would be no heavy sludge removal tankers to traffic them.
LAS enlisted MWH to undertake construction of the plant.
Final outturn cost at Errol was £1.6M and the plant was delivered impressively fast - construction took just nine months.
If there is a drawback, it is the 20 day treatment period. Conventional treatment works can turn sewage around in just eight hours. Wynnes concedes that this time lag means the system is not suitable for all areas: a large population would need a huge set of ponds to prevent the system being overloaded - a minimum surface area of 2m 2per head of population is needed to provide the correct conditions.
But the issue of land take is becoming less of a sticking point, says Wynnes, as water companies begin to look for more sustainable solutions to sewage disposal. And with agriculture in crisis, farmers are increasingly keen to earn revenue from alternative activities. Taking out long term leases on farmland is becoming more and more financially viable.
Errol has attracted attention.
Since being commissioned, MWH and LAS have been approached by several other water companies interested in building a system of their own, Wynnes reports.
Most recently, one firm wants to build a biological system for a population of 100,000 on a 25ha site. Errol, therefore, may be the tip of a sustainable sewage iceberg.
INFOPLUS MWH: www. mwhglobal. com.
Contact construction services manager Ian Wynnes, tel 01494 526240, e-mail ian. wynnes@mwhglobal. com.