Currently having reached stage 2 of the upgrade process, which involves the construction of the Sludge Management process, the upgrade of Zeekoegat is well under way, discovers Chantelle Mattheus on consultation with principal for Water & Wastewater Treatment at Bigen Africa, Ian Pollard.

With construction of the new 40 Mℓ/d activated sludge treatment module under Stage 1 nearing completion at the City of Tshwane’s Zeekoegat Wastewater Treatment Works (WWTW), construction of the new sludge management and treatment process under Stage 2 commenced in November 2012. Three months into the 20-month contract, the Contractor, a joint venture (JV) between Civcontract Civils and WEC Projects, has made good progress and the majority of earthworks have been completed on this R188 million contract, reports Pollard.

“The objective of the new infrastructure is to provide a comprehensive sludge handling, treatment, stabilisation and dewatering process for the Zeekoegat treatment facility as a whole, and it is envisaged that approximately 8.5 t primary sludge and 15 t waste activated sludge (WAS) will be routed and treated by the new process in the medium term,” he says.

Process unpacked

Pollard adds that primary sludge captured by the primary sedimentation tanks will be pumped to two new fermenters, where the sludge will be retained for a period of three tosix days for the formation of volatile fatty acids to enhance biological phosphorus removal at the works, before being pumped to one of two new 6 000 m³ anaerobic digesters for stabilisation. WAS extracted from the biological reactors and thickened by means of dissolved air flotation will be deaerated and pumped directly to the anaerobic digesters, which will be completely mixed units designed to operate at 16 days hydraulic retention time and 35˚C to facilitate the formation of biogas. This will in all likelihood be utilised in the nearby future for the (co) generation of electricity.

“Based on the expected production of biogas and specifically methane, it is envisaged that between 800 and 1 200 kW electricity, which is roughly 30% of the plant’s total electrical demand, can possibly be generated at the works and routed back into the local electricity supply grid,” describes Pollard. As a result, in the short-term, the biogas will be used as fuel for two hot-water boilers, which will be used to heat the contents of the digester through a series of heat exchangers.

Additionally, digested sludge will be routed to a new dewatering facility where the sludge will be dewatered by means of belt filter presses to a solids content of approximately 20% before being spread on a 30 000 m² concrete slab for solar drying prior to being made available to an external party for the production of fertiliser. The provision of all sludge loading, spreading and turning equipment has been included under the scope of this contract.

“A further feature of the sludge management process is the treatment of phosphate-rich liquor discharged from the digested sludge during dewatering. This liquor stream will be chemically conditioned by means of lime for the precipitation of soluble phosphate, with the resultant chemical sludge being routed back to the belt filter presses for drying and subsequent disposal,” adds Pollard.

The project is being managed for the City of Tshwane by Izak de Villiers, Koot Snyman and Stephan van der Merwe. The design and contract administration for this stage of the project is currently handled by Corrie Marx and Ian Pollard of Bigen Africa Services as part of the BAKV3 JV between Bigen Africa and Kwezi V3 (now Worley Parsons). As mentioned earlier, the contractor is a JV between Civcon and WEC Projects, with Electron Technologies as the electrical subcontractor.

During the peak of civil construction activities between March and September 2013, between 70 and 100 people will be employed on-site as general workers, steel fixers and shutter hands, many of them employed from the local community.

“In addition, the majority of the building works, fencing, paving and small bore pipework will be subcontracted to local emerging contractors as part of the contractor’s local economic development initiative,” concludes Pollard.