Water treatment solutions company Veolia Water Technologies South Africa earlier this month completed the installation of an anaerobic digestion pilot plant designed to test the digestibility of the domestic and industrial waste received by eThekwini Water & Sanitation’s southern wastewater treatment plant in Merebank, Durban.

Veolia Water Technologies South Africa pro-ject engineer Herbert Kleinhans notes that Veolia has entered the commissioning phase, which is expected to be completed by the end of this month.

The success of the pilot trial will be assessed in terms of the effective generation of the biogas through the anaerobic digestion relative to the wastewater-feed flow rate from different sources, he adds.

Following commissioning, Veolia will operate the plant for the first 12 months, after which the client will operate the plant and continue testwork for an additional timeframe.

Veolia’s scope of work for the eThekwini munici-pality included the supply, construction and com-missioning of flow tanks, primary settling tanks, thickeners and digesters, as well as suction and delivery pipework, pumps, electrical power and control instrumentation for the digestion plant.

The company was also responsible for the mechanical and electrical design of the plant, while project-appointed consultant Hatch is responsible for the process design.

Once in operation, Veolia will implement a one-year operation and maintenance contract, during which time the company will work with research students from the Pollution Research Group at the University of KwaZulu-Natal, who will gradually assume operation of the pilot plant.

Digestion Process
Because of the varying waste composition and feed rates to the plant, piloting quality-specific treatment processes was required to confirm the efficacy of the anaerobic digestion process to generate biogas from the current wastewater feed, Kleinhans explains.

The plant will concentrate sludge through settling and thickening, followed by anaerobic digestion.

Raw domestic and industrial sewage extracted from the Chatsworth, Badulla and Jacobs sewers feeding the treatment plant will be fed into three flow tanks (one tank for each inlet line) at a continuous flow rate of about 14 m3/h.

From these flow tanks, 200 m3/d of raw sewage will be pumped to two primary settlement tanks, in proportion to the flow rate of the sewer feeding the tank.

The primary settlement tanks form the starting point of a two-train treatment process.

Raw sewage extracted from the Chatsworth sewer will feed train one, while the Jacobs and Badulla sewers will provide the feed for the second process train. Underflow from the primary settlement tanks will be transferred to thickeners, with each one receiving about 2.5 m3/d of intermittent feeds.

Settled sludge from the thickener underflow will periodically be drawn off and fed into the 10 m3 digesters. The biogas generated during this digestion phase will pass through a flow meter, gas analyser and flame arrestor before being released into the atmosphere.

On successful completion of the pilot period, and should the biogas production be proven viable, eThekwini Water & Sanitation might decide to use the data gathered as the design basis for a full-scale plant, Kleinhans concludes.