To boost the town’s sewage treatment capacity, the Walvis Bay Municipality is upgrading its filter technologies at a local sewage treatment works.

This upgrade, valued approximately at ZAR 15 million, will significantly improve the quality of final effluent from this portion of treatment, amounting to about 6.5 mega litres per day. Existing civil works in the form of the old trickling filters’ superstructures were rejuvenated to house the new generation treatment process.

The municipality has contracted ASE (Aqua Services & Engineering), a subsidiary of the global Veolia Water Solutions & Technologies, to upgrade existing stone-media biofilters to the company’s flagship trickling filter technology.

“It’s the first time in Namibia that the new-generation trickling filter technology was used to replace old stone filter media,” says Chris Stöck, Managing Director at ASE. “Trickling filters give far fewer operational problems and, with regard to blockages and ponding, are decades more advanced.”

At full operation, the new polypropylene (PP) media will host the micro-organisms that, in stage one, remove carbonaceous material such as COD and BOD (chemical oxygen demand and biochemical oxygen demand). In stage two, the micro-organisms perform nitrification. The effluent is continuously recirculated through the PP media at a high rate, increasing the biofilters’ aeration by three times which, in turn, multiplies the biological treatment’s efficacy three-fold.

With an effective surface area of up to 150 m² per m³, the media is specially designed to facilitate fast growth of microorganisms on their surfaces, while the large passage size and cross-flow patterns effectively prevent blockages and ponding.

“ASE upgraded two of the three existing biofilters by packing the larger 45-metre diameter filter to heights of 3, 6-metres with new PP media that’s designed specifically for stage one processing. The smaller 30-metre diameter biofilter was packed 1.8-metres high with PP media suitable for stage two processing, for an effective total of 7 000 m³ of PP filter media,” says Stöck.

ASE imported the media as individual sheets and, once in place, welded them together using a specially-imported German fusion welding machine. The assembled blocks are self-supporting, and avoid creating unnecessary pressures on the biofilters’ side walls, making them ideal for use in older plants.

As part of the upgrade, ASE was also responsible for installing the plant’s new recycle pump station.  Specified for construction three metres below ground level, ASE faced a unique challenge to overcome the presence of groundwater and its high salinity. “Normally, we find groundwater to be relatively fresh but, because Walvis Bay is a coastal town, the groundwater level is very shallow, and the water very salty,” says Stöck.

Due to this and the proximity of existing structures, ASE decided to use sheet-piling and dewatering pumps to excavate for the pump station, and the concrete pump station was then constructed in situ by conventional methods. ASE also supplied and installed new stainless steel biofilter distribution arms to replace the existing biofilter arms, which were already in disrepair and not sized for the increased recirculation rate.

The newly constructed plant and pump station is currently receiving in excess of 500 m³ per hour of raw sewage at peak flows, and the total continuous recirculation flow through the filter media is approximately 1 240 m³ per hour. The system is undergoing performance testing to ensure effluent quality.

Walvis Bay is the latest of a series of towns in Namibia to kick start a large scale water infrastructure upgrade, following the construction of a new Swakopmund wastewater treatment plant, of which ASE supplied mechanical works, and an upgrade to the Katima Mulilo plant that will feed drinking water into the Caprivi pipe system.