Water and Effluent Treatment - Where there's sludge, there's money06 December 2004

A new sewage treatment works sets new standards for reduction of pollutants in its effluents, its odour-free operation, easy and safe access for maintenance operators, and attentions to the requirements of new regulations such as ATEX.

Since all these requirements are likely to become more stringent in future rather than less, its present high level of performance is likely to be superseded in future. But this prospect holds no fears for the designers and builders, who give the impression that they quite look forward to the next challenge.

The £70m Reading sewage treatment plant recently completed by the Black and Veatch Alliance Joint Venture with Taylor Woodrow for Thames Water is truly state of the art. It is designed to treat 1800 l/s FFT (full flow to treatment) and 4000 l/s peak, yet requires only two operators plus a small number of maintenance staff to work it. The final effluent has to contain no more than 5mg/l BOD (biochemical oxygen on demand), as opposed to the 25mg/l that would have been considered acceptable a few years ago. Nitrogen as ammonia has to be no more than 2mg/l and TSS (total suspended solids) 30mg/l. Annual total phosphorus is 1mg/l. Output sludge, sold as fertiliser, has to be free from Salmonella and treated to ensure a 6 log (99.9999%) reduction in pathogens. And because the plant is near a major town, it must not smell, with a requirement of no more than 98 percentile 5 OU (odour units) at the plant boundary. This means that the air should only have to be diluted five times for a smell to be at the limit of human detection.

This is achieved by covering and extracting air from above tanks and channels that are normally open to air in a sewage works. Others remain open but are inside closed buildings. Because channels and ducts are closed, they have to have hydrocarbon monitors, especially since the plant came into operation when ATEX was already in force. Inside some buildings, there is a slight smell, but air from these is also extracted. Extracted air is chemically scrubbed using sodium hydroxide and sodium hypochlorite, with 100% regenerable active carbon filters on standby. All buildings have hydrocarbon and hydrogen sulphide monitors.

Being shown round by Black & Veatch principal process engineer, Dr Patrick Coleman, it is striking to see a process plant where all pumps are accessible in the open. Modern health and safety requirements, as well as ease of maintenance, require that pumps should not be buried under tanks or sunk in trenches.

The primary method of breaking down the unwanted organic matter, as in all modern treatment plants, is to make use of microbes working in activated sludge, some of which is removed from the final settling tank and returned to the start of the main treatment process. This consists of a fermentation stage, an anaerobic zone (no nitrate, no oxygen), an anoxic zone (nitrate, no oxygen) and an aeration zone.

The flow sheet is based on the process invented by Dr James Barnard, senior process director at Black & Veatch. His first findings on biological phosphorus and nitrogen removal were published in 1974 and patented by the National Institute for Water Research (NIWR) in South Africa under the name Bardenpho (Barnard denitrification and phosphorus removal). In the flow sheet at Reading (see overleaf), the fermentation zone reduces dissolved oxygen and nitrate in the returned activated sludge and also ferments one of the main inputs to the plant, the effluent from the neighbouring Scottish Courage brewery. The anaerobic zone is there to force the bacteria in the returned sludge to give up their phosphorus. The released phosphorus comes from adenosine triphosphate (ATP), which is a stored energy form inside the bacterial cell. The bacteria break the phosphate bonds of the ATP in order to obtain enough energy to absorb the BOD.

In the anoxic stage, the bacteria take up nitrates in order to take up BOD, while in the aerobic stage, the bacteria oxidise the stored BOD and also take in phosphorus to rebuild their ATP.

The surplus sludge is blended with sludge removed from the initial raw sewage and goes to pasteurisation and digestion in large, onion-shaped vessels. Pasteurisation, at 70°C for one hour, has the effect of making the sludge more digestible. The digestion process produces the largely methane biogas which is stored in inflatable balloon storage tents. It then goes to a combined heat and power system which produces half the energy consumed by the plant.

Complications include the presence of siloxanes derived from modern cosmetics that can crystallise in the gas, wrecking engines. They are dealt with at Reading by dosing with a proprietary polymer.

Dr Coleman makes the point that what works at one plant in one country does not necessarily work at another, because sewage has different compositions on each site. He cites as an example the fact that at one point no sludge dryer was working in the UK because our sewage contains more fibres than the continental sewage for which the dryers had initially been designed.

A particular problem at Reading, because of hard water, is the possible formation of struvite (magnesium ammonium phosphate) in pipes and on impellers. However, it is also an opportunity because struvite is used elsewhere in the world as a fertiliser. Based on their experience at an earlier plant at Slough, Thames Water Research and Technology Group contributed a number of changes to the Reading design to minimise struvite formation. They also gave a description of the necessary additions for a future crystallisation plant, should a market in the UK become established; space has been left in some of the buildings for the extra equipment.

Dr Coleman reports that the final performance of the plant is well within design target parameters, producing 0.06 mg/l of nitrogen as ammonia in the final effluent going ultimately to the River Kennet, plus 1 mg/l phosphorus and 1-3 mg/l BOD.

He predicts that targets for effluents are likely to become ever stricter, and already are so at some US sites. Pesticide and prescription drug residues, not presently addressed, are also likely to be issues in the near future. Nonetheless, he takes the view that if customers were to demand more stringent targets, his company would look forward to meeting the new challenges, as long as the customers accepted the inevitable additional costs.

SOE

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