Welgedacht Waste Water Treatment Works – Module 2 Extension

All this necessitated the building of a Module 2 extension to cope with increased demand for wastewater treatment from Benoni, Boksburg, Springs, Bakerton and Daveyton. The client required an additional 50 MLD and the new Module 2 extension was designed by a consulting engineering company.

Description of Project

WEC Projects was part of a joint venture that was contracted to build the mechanical and electrical scope of work for the new Module 2 upgrade at a total contract value of R160 million. The civil contractor was appointed on a separate contract. The project was awarded towards the end of 2012 on an initial 26 month contract. Construction was only completed in mid-2016 due to some delays in the civil construction contract as the geographic location is renowned for unfavourable and unforeseen dolomitic soil conditions and a very high water table. This proved challenging in terms of the civil and earthworks construction. Commissioning, training and handover ran into early 2017.

Process Outline

The process consists of inlet pump station, inlet screening works and degritting, balancing tank, primary settlers, reactor and aeration equipment, secondary settlers, ferric, lime and chlorine gas dosing, tertiary sand filters, sludge holding tank, anaerobic digesters with a double membrane gas holder and gas flare for the methane generated in the anaerobic digesters and all associated electrical, electronic and control equipment. The tertiary rapid gravity sand filters were included as per the engineer’s design. These are monolithic type filters with false floor, filter nozzles, sand media and air scour blowers and backwash pumps. Additional work was added to the joint venture’s scope in the form of an electrical substation with 22 kV ring feed. For a detailed description of the process please contact us.

Process Description

Pump Station:  The upgrade consisted of an archimedes screw pump lifting station at the inlet of the wastewater treatment works. This pump station splits the flow to Module 1 and the new Module 2. In module 2, the raw sewage is then passed through a set of manual bar screens and mechanical front rake screens and degritted in a bank of vortex degritters. The flow is monitored with a partial flume The inlet works is complete with screw conveyors and screw compactors for the screenings and grit classifiers for the grit washing and removal.

Balancing tanks:  Two balancing tanks were constructed for peak flow management. The balancing tanks required mixing and an additional screw pump station in each tank to transfer the screened and degritted wastewater to the primary settlers.

Primary settler tanks: Two conical bottom primary settler tanks with periphery driven half bridges allow for the removal of primary sludge. The settlers are complete with stilling wells, overflow v-notch weirs and sludge collection chambers. The primary settled water is then sent to the biological nutrient removal (BNR) reactors. The primary sludge is directed to the primary sludge handling facility consisting of sludge screens, hydro conveyor fluidisation pumps, sludge transfer pumps and primary dewatering presses.

BNR Reactors:  The two BNR reactors are designed for the JHB process and includes JHB anoxic zone 1, anaerobic zone 1 and 2, anoxic zone and an aerobic zone. All anoxic and anaerobic zones are fully mixed with surface mounted mixers and the aeration zone is fitted with blowers and fine bubble diffusers.

Secondary settler tanks:  The mixed liquor then flows to the secondary settlers. Four conical bottom secondary settler tanks with periphery driven half bridges allow for the removal of secondary sludge. The settlers are complete with stilling wells, overflow v-notch weirs and sludge collection chamber. The clarified water is diverted (via an archimedes screw pump station) to a ferric flocculation channel and dosed with ferric chloride and lime. Return activated sludge (RAS) on each reactor is achieved by dedicated RAS archimedes screw pump station. Waste activated sludge (WAS) is pumped to the sludge handling train.

Tertiary treatment system:  The effluent is passed through a tertiary treatment system of rapid gravity sand filters. The sand filters include false filter floors, nozzles, sand media, blowers, backwash pumps and actuated valves for automatic backwash. The filtered water is then disinfected via a chlorine gas dosing system complete with motive water pumps, dosing and safety equipment and a chlorine contact channel. The treated effluent is then sent to the maturation ponds.

Sludge handling train: The sludge handling train consists of two anaerobic digesters with mixing, sludge pump station, sludge tank and heating system (via LPG boiler and heat exchangers). The gas from the anaerobic digesters is collected and in a gas holder for use in the boiler or is flared.

Challenges and solutions

The blower selections and fine bubble diffuser network required some design refinement and took additional time for approvals from the engineer. This was solved successfully and resulted in a world class and efficient aeration system installation.

There also were some initial challenges during the defect liability period on the operation of the sand filters and the inlet works. However, these were solved by WEC Projects providing an additional extensive operation and maintenance training programme with extended support visits and assistance.

Results

• The plant has now been successfully running for five years with the client and engineering team very satisfied with the installation
• The treatment works is no longer a concern for the surrounding communities as a result of the wastewater works no longer being overloaded