Boosting efficiency of effluent filtration

Copa has launched a moving-bed sand filter, which the company believes offers a new low-head alternative for tertiary filtration

The final determination of the next periodic review, PR04, is with us and many plants will require updating to meet new effluent quality requirements. Tighter consents for suspended solids and biochemical oxygen demand (BOD) require additional tertiary filtration systems that meet the new standards, are reliable, easy to maintain and are cost-effective.

In order to satisfy this need, Copa, in conjunction with Organo UK, has recently introduced the Toveko moving-bed sand filter into the municipal wastewater market. Efficient sand cleaning is critical to maintaining optimum filter operation, especially in wastewater applications where biological growth can also occur on the media, increasing resistance to flow and reducing treated water output. With many filters the sand is washed in-situ with water and air scour backwash. Here, the air and water flow rates and wash duration govern the cleaning efficiency. Other filters are continuously washed by moving sand from the bed, cleaning it hydraulically or mechanically, returning the clean sand to the filter bed. These are known as moving-bed filters and Copa believes they have many advantages over more conventional systems.

Sand filter trials

A Copa Toveko tertiary sand filter has been undergoing extensive trials at Southern Water's Wateringbury WwTW, treating the effluent from the trickling filter humus tanks. The plant comprises a low head, upflow moving-bed filter, with an external sand wash. The incoming effluent flows into a longitudinal inlet shaft and is distributed evenly across the base of the filter bed by a series of inverted V-notch openings along the length of the filter. The feed flows up through the bed, with filtered effluent flowing out from the top. The dirty filter sand is continuously removed from the base of the bed with an air-lift and transferred to an inclined screw sand washer located above the bed.

The screw moves the sand through a counter-current flow of wash water, with cleaned sand spread on top of the filter by the traversing action of the sandwash unit. Dirty wash water flows away to drain. In periods of high flows or increased solids loading, the rate of sand turnover and washing increases automatically. As the head increases, the air supply to the airlift pump increases, and hence the quantity of sand transferred to the washer and the speed of the washer screw are increased to ensure efficient bed cleaning.

Performance measured

Features that set the Toveko filter apart from more conventional options are its low head, only 2.3m, efficient sand wash, the ability to respond to changes in flow and load and simple maintenance and operation. During the initial month-long steady state trials the filter was operated at a filtration rate of approximately 8-9m/h. Samples of influent, effluent and backwash were taken daily and analysed by Southern Water for ammonia, BOD, total and soluble, suspended solids (SS) and turbidity. The results for % BOD and SS removal are shown in Figure 1.

During the period of the trial the filter averaged more than 60% removal of suspended solids, with average concentrations reduced from 25mg/l to, in most cases, significantly less than 10mg/l. Total BOD removal averaged 44%. However, filterable BOD removal averaged more than 65%, indicating good filtration performance. Further trials were carried out to investigate the performance of the filter, with increasing hydraulic and suspended solids loading rates.

Samples were taken approximately 20min after each increase in flow or load and analysed for the same parameters as in the steady state trials. For the hydraulic trials the filter was operated at filtration rates of 6.3-16.7m/h. The results show a slight reduction in SS removal with increasing filtration rate, however, SS removal still averaged a good 56% over the trial, even though the higher filtration rates exceeded the design maximum of 15m/h.

In the solids loading trial, trickling filter effluent was added to the filter feed to increase the solids loading. Solids loading rates were increased from 0.2-1.3kg/m2/h, with an average of 0.6kg/m2/h. Suspended solids and BOD removal averaged 77% and 63% respectively and the results show a correlation between applied and filtered suspended solids load, shown in Figure 1.

Gauging reaction

During the operation of the filter trials by Copa, the plant has worked reliably with minimal routine maintenance. A series of open days have been held to introduce the filter to consultants, engineers and operators in the industry, demonstrating the sand wash carriage, and the performance and reliability of the plant.

Nina Downes, process scientist with Southern Water, said: "I particularly like the visibility of the sand washing on the Toveko filter. You can see at a glance that sand is being delivered to the washer and solids are being removed. The clear display of differential head is also a benefit, allowing easy monitoring of sand filter performance."

Alan Smith, Copa group process manager, said: "We have been very pleased with the results of the Wateringbury trial. The Copa Toveko filter has an extensive reference list worldwide, mainly in the industrial market, where its ability to handle 'sticky' solids is well recognised. Further trials are planned with other water companies to demonstrate the advantages of the filter in tertiary wastewater applications."


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