Economics driving new mini

Many water companies must feel they are working in an environment in which the goalposts are constantly being moved. Just as they comply with one set of standards, the standards tighten up. Tighter controls on BOD, suspended solids (SS) and now phosphorus are the most common. It can mean that a treatment works which has previously been operating effectively to achieve earlier standards has to be upgraded.

In several instances the company has made use of Tetra deep bed filters to achieve the desired results. Tetra deep bed filters rely on a thick layer of carefully graded sand – 1.2-1.7m deep – over a layer of gravel, supported on the firm’s patented T-blocks. The T-blocks support the filter medium and provide a floor for the backwash system.

These deep bed filters have proved highly effective in improving the quality of effluent but they are designed to handle large volumes, and in their standard design, require fairly substantial civil engineering work during construction. However, STW faced much tighter consent levels for BOD, suspended solids and in some cases phosphorus at some of its smaller works. Tertiary treatment to polish the final effluent would be needed to meet the new standards. The question was, what type?

In small doses

STW liked the Tetra system and was satisfied with the performance of the Tetra process plants at its larger works. However, when the company considered constructing similar installations to process smaller volumes the costs proved much higher than those of competing systems. The manufacturers of the Tetra process, Severn Trent Services, therefore began to look into ways and means of translating the technology into a more appropriate form for treating smaller flows.

What the firm’s engineers came up with is a modular package design. In this mini version the medium is contained within a circular cell formed of stainless steel, 1m in diameter. Each cell contains 1.2m depth of filter medium, with a freeboard of 2m above. Each has its own T-block floor and underdrain system.

The cells can be arranged in groups of 6, 8 or 12, depending on the requirements. Any cell can be taken out of service for backwashing without seriously affecting the quality of the effluent. The flow is automatically split between the remaining cells and is adjusted as required.

The modules are self-contained and skid mounted. All that is needed on-site is a concrete slab to act as a base – for an 8-cell structure this would be 9 x 5m, and 11 x 5m for 10 cells, etc. Once on-site all the units require is pipework connections and a link to the power supply. Delivery to full operation may take as little as a day.

So far Severn Trent Water has installed three of these compact mini units at treatment works where tertiary treatment is now needed
to meet reduced consent levels. Hodsock and Shirebrook have dry weather flows roughly similar at around 2,300m3/day. Consent levels for Hodsock were cut to 15 BOD, 40 SS and 5 for ammonia, while the comparative figures for Shirebrook were 20 BOD, 40 SS, 5 and 10 for summer and winter ammonia levels respectively. Both, however were required to keep phosphorus at no more than 2mg/l.

Cuts in consent levels for the third site, which is treating influent from the village of Broadway, were even more severe – BOD has been reduced to 10 and SS to 20, with the figure for ammonia levels cut to 5. Broadway
does not have a phosphorus content, but it does have a fluctuating population, with coach-loads of day-trippers and overnight visitors during the tourist season which can affect the flow considerably.

All three sites have been running the Tetra modular deep bed filters for around a year. All the units have produced excellent quality effluent, according to Alison Fergusson of STW’s process engineering department. “There were a few of the usual teething problems during commissioning, but the only major hiccup was when the air compressor which operates the pneumatic valves was stolen from one site. I understand the design has been modified to prevent this happening again.”

Howard Rundle of Severn Trent Services believes the Mini Tetra units have a great future. “The system is factory constructed and assembled, which means quality control is excellent. It is very flexible – you can add cells, or even whole modules as needed, and we are looking into the possibility of wheel-mounted versions to deal with temporary situations. A module takes up little space, is quick to install and, of course, it brings an already established and well proven technology within the reach of smaller works at an economic price.”

Even more important, results from the three sites have been excellent. Samples of final effluent tested by the EA show average effluent SS of 9.1mg/l at Hodsock, 4.2mg/l at Shirebrook, and 2.3mg/l at Broadway.

With this record behind them the new Mini modules could be the answer for a vast number of works where operators are finding they are having to meet tighter standards than those for which the works were designed.