Umgeni Water flagship project nears completion

Constructed in the mid1960s as a pump station, with the addition of biofilters in the 1970s and biological processes in the 1990s, the Darvill Wastewater Treatment Works (WWTW) has always been an engineering feat. The fourth upgrade is nearly complete, having continued the tradition of embracing innovation.

By Kirsten Kelly

The Darvill WWTW is owned and operated by Umgeni Water and serves Msunduzi Municipality, receiving and treating both domestic and industrial wastewater from the city of Pietermaritzburg in KwaZulu-Natal. Its original design capacity was 65 Mℓ/day, which has been upgraded to 100 Mℓ/day, with plans for a further 20 Mℓ/day extension in future.

Steady increases in the hydraulic load, a 33% spike in the organic load in recent years, a population of around half a million, and a reduction to the plant’s discharge limits prompted the current upgrade. “Currently, different industries are supplying wastewater to Darvill. The upgrade has therefore given us the opportunity to install robust processes that can deal with the effluent and different contaminants from these industries,” adds Lindelani Sibiya, project manager, Umgeni Water.

Sustainability

has been upgraded to include two sustainability projects: • Methane generated by the anaerobic digesters (currently as boiler fuel) may be directed to a gas-to-electricity cogeneration plant. The project is at feasibility stage. • A 2 Mℓ/day direct reuse plant is in the final stages of commission. “This is a flagship WWTW for Umgeni Water due to its size, the iconic eggshaped digesters and, more importantly, its processes that embrace a circular economy. Bulk water is our core business, and we therefore focus on maintaining and improving the quality of water throughout the water cycle. Catchment management is extremely important. We have to treat our wastewater properly so that, when it is discharged back into the catchment, we can use that water again,” explains Megan Schalkwyk, process engineer, Umgeni Water. Umgeni Water is focused on reducing waste, keeping resources in use for longer, and limit4ing the use of additional natural resources.

Sludge disposal Currently, sludge generated by the anaerobic digesters is directed to an external commercial enterprise for turf grass manufacture. This is a sustainable option for the solid waste generated at the plant. Umgeni Water is investigating the feasibility of installing an electricity cogeneration process (combined heat and power). The most unusual structures on the project are the Megan Schalkwyk, process engineer,

Umgeni Water Lindelani Sibiya, project manager, Umgeni Water

At Darvill WWTW, Umgeni Water is gradually implementing projects to make a circular economy more achievable

egg-shaped digesters. The upgrade includes the construction of two additional 36 m high, 18 m diameter egg-shaped digesters. These ‘concrete eggs’ are the third and fourth of their kind in South Africa – their predecessors being the existing two built in 1975.

Approximately 25 000 kg/day of sludge is fed into these digesters, with methane-rich gas emitted during the digestion process. This gas shall be used to generate electricity (for Darvill WWTW) through a process referred to as cogeneration, yielding 800-1 000 kW of electricity per day.

Water reuse demonstration plant

Additionally, a new 2 Mℓ/day water reuse demonstration plant was designed, which will treat the final effluent to potable standards. The plant incorporates several advanced technologies – such as advanced oxidation and biologically activated filters and ultrafiltration membranes – thereby providing multiple barriers against contaminants of emerging concern such as nanomaterials, pharmaceuticals and endocrinedisruptor breakthrough.

Initially, it will be used as process water and provide opportunities for further research into complexity, efficiency, life-cycle costs and adaptability of water reuse in the South African context. It will be a valuable example of reclamation in practice to stakeholders. Bubble diffused aeration system

The biological treatment system has been changed from a modified Johannesburg system to a three-stage Phoredox biological nutrient removal system. This treatment process has been optimised by adding a bubble diffused aeration system that is more energy efficient than surface aerators. The fine-bubble diffused aeration system supplies oxygen to the metabolising microorganisms.

“The 7 m deep aerobic reactor is used (owing to space constraints), which has resulted in an increase in the oxygen transfer efficiency for the treatment process. This system allows for smaller air bubbles to be introduced into the reactor, creating a larger surface area for the mass transfer of oxygen into liquid. This reduces power use while increasing efficiency,” says Schalkwyk.

Challenges

One of the main challenges with this upgrade was the phasing in and out of unit processes and deciding what to decommission first. The plant was already under strain due to capacity issues, so this meant reduced capacity over that period.

“At times, we had to make decisions to forgo compliance for a while, in order to allow the contractor to shut down existing infrastructure, while at the same time the inherent challenges of operating Darvill continued, such as trade effluent illegal discharges

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SUMMARY OF UPGRADES TO THE DARVILL WWTW

• Primary treatment - An additional 40 m diameter primary settling tank with a surface area of 1 250 m2 • Biological treatment - Conversion of existing modified Johannesburg process to a three-stage

Phoredox BNR system - Construction of new reinforced concrete aerobic reactor (40 150 m3) • Air for biological treatment - Blower house to provide air supply for a fine bubble diffused air aeration system - 4 x 645 kW blowers each with a rated delivery of 7 m3/sec @ 90 kPa - Air header mains - 14 256 diffusers in four aeration lanes • Secondary treatment - 2 x 35 m diameter secondary settling tanks - New chlorine house and scrubber system • Advanced treatment - Conventional water treatment works - Ultrafiltration and advanced oxidation system

and unreliable power supply,” says Mulalo Murigwathoho, systems manager, Umgeni Water. The rainy season was worse, as it meant delays on construction, guaranteed process failures, and complaints from the canoeing community. A number of directives were issued during the course of the upgrade, because of some of these failures.

Later on, the main contractor abandoned the contract and work stopped for approximately two years, while processes to appoint another contractor were in progress. That caused the employer to take on the operation and maintenance of incomplete processes and operations, and remaining with infrastructure no longer under warranty and guarantee. Some of this equipment had never been operational and Umgeni Water had to take on the risk to rehabilitate later on. The project suffered a significant setback when the main contractor filed for business rescue.

“In order to ensure work continuity, Umgeni Water applied for consent from National Treasury to negotiate with subcontractors on the project to complete the remaining works. Consent was granted and later partially implemented in favour of competitive bidders to control costs for the new main contractor. The new main contractor resumed construction work in August 2020 and is expected to complete it by end October 2022,” states Sibiya.

Covid-19 was another challenge that caused delays and financially impacted smaller subcontractors, making it difficult for them to complete work.

There were additional challenges faced by the project due to complexities on the designs, integration of the old and new units, adverse climatic conditions and their implications on the ground, to mention but a few. “Despite these challenges, Umgeni Water is extremely proud of this project. It is proof of our commitment to our environmental sustainability policy that aligns with circular economy principles. This project also affirmed Umgeni Water’s strength and resiliency in bulk water infrastructure development. It changed many lives for the better. Small, medium and micro enterprises were also contracted to perform on the project. The contractors tried to use as much local labour as possible and empowered them with new skills and experience. A number of graduates were trained through this project as well,” Sibiya concludes.

PROJECT STATISTICS

• Over 800 kW electricity will be generated from the plant’s methane gas by-product • Two new digesters were installed, doubling capacity

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