The success of the Gansbaai wastewater treatment works (WWTW), designed by engineering consultancy firm SSI Engineers & Environmental Consultants, a subsidiary of the Netherlands-based DHV Engineering Group, in 2009, was a deciding factor in constructing another Nereda plant in Epe, in the Netherlands, DHV water inno- vation and product development manager Andreas Giesen tells Engineering News.
“We learned from the Gansbaai design and used this knowledge to optimise the design of the Epe plant,” he adds.
The Epe plant became operational at the end of 2011 and was officially inaugurated by the Crown Prince of the Netherlands, Willem-Alexander, on May 8.
The plant treats wastewater, equivalent to that produced by 60 000 people, from the municipal sewer system at a flow rate of 1 500 m3/h. About 15% of the influent comes from slaughterhouses, says Giesen.
“This water is treated to comply with Europe’s standards for swimming water, as it is discharged into a creek that flows into a lake,” he says.
This meant that the total nitrogen concentration in the water had to be reduced to less than 5 and the phosphate levels to less than 0.2, as the quality of the water is largely deter- mined by the extent to which nitrogen and phosphate levels in the water are decreased.
Further, Giesen points out that the Netherlands is a water-rich country and, therefore, while water does not need to be considered for reuse, as is currently the case in South Africa, it does need to meet high discharge requirements. “Our aim is to keep the water quality in the creeks and rivers at a good standard, which is why we treat our wastewater to these levels,” he explains.
The plant in Epe is the first Nereda plant in the Netherlands to treat municipal wastewater.
A few Nereda pilot plants and full-scale plants for industry were operated in the Netherlands before the Gansbaai WWTW was constructed, says Giesen.
“As the technology matured, we were looking for a municipal site to validate the scale-up to larger capacity.”
At that time, the Overstrand municipality was planning to upgrade the Gansbaai WWTW. The municipality compared numer- ous technologies and found Nereda to be attractive.
Overstrand municipality also successfully managed the risk relating to the new technology, Giesen says.
The Gansbaai WWTW was the world’s first full-scale domestic sewage treatment works to use the new technology in a R25-million upgrade project.
The plant produces high-quality effluent using a standard two-million-litre-a-day sequential batch reactor designed to treat 4.5-million litres of water a day. This efficient approach has eliminated the need for the new five-million-litre-a-day works planned by the Overstrand municipality, achieving substantial cost benefits, SSI principal specialist Paul Gaydon says.
“Capital costs are some 25% lower and oper- ating costs about 30% lower than than those of equivalent conventional systems,” he adds.
Nereda technology, developed by the University of Delft, in the Netherlands, in conjunction with SSI’s parent company, DHV, purifies wastewater through an aerobic granular biomass process.
This treatment process consistently produces high-quality final effluent suitable for environmental discharge or irrigation.
Further, with additional polishing steps, the final effluent can be processed to potable standards, says Gaydon.
Wastewater treatment works generally use an activated sludge process, which brings bacterial sludge into contact with sewage. The bacteria use the wastewater as a food source and the nutrients (contaminants) for cell maintenance and growth.
Activated sludge is retained in suspension by mechanical mixing or stirring, which maintains contact between the sludge and the wastewater, which is essential for water treatment to occur.
Once the wastewater has been treated, the blend of water and sludge is discharged to a settling tank, where the sludge is allowed to settle out of suspension and the clear liquid is discharged for disinfection. The settled sludge is then returned to the reactor to restart the treatment cycle.
The Nereda system also comprises an activated sludge process containing the same basic bacteria; however, the configuration process encourages the bacteria to form in dense colonies, called granules, Gaydon explains.
Sewage is fed into the Nereda tank with the granules and is aerated. The aeration provides the necessary oxygen and mixing (as the bubbles rise to the surface, they give buoyancy to the granules) to ensure contact between the granular sludge (bacteria) and the sewage.
Through this process, the water is cleaned within two to three hours. At this stage, the aeration is stopped and the granules settle to the floor of the tank. The top water is discharged as final effluent during which time the next batch of sewage is fed into the bottom of the reactor. The aeration is then restarted and the cycle is repeated.
The dense, round granular structure allows fast sludge settlement, which allows Nereda to operate at sludge concentrations of two to three times greater than that of conventional floc-type activated sludge systems, without the settling limitations of conventional sludge systems.
“The higher concentration of bacteria increases the amount of wastewater that can be handled for each unit of tank volume,” adds Gaydon.
While conventional water treatment processes require separate zones, with recirculation pumps to transfer the sludge and blend water between zones to enable the removal of biological nutrients, Nereda’s granular structure allows the zones to form inside the granule so that no recirculation pumps or separate civil structures are required for biological nutrient removal.
This results in Nereda’s environmental footprint being generally 50% smaller than conventional wastewater treatment systems and it requires less capital than would be needed to build a conventional treatment system.
Further, the amount of mechanical equipment required for a Nereda plant is much less than required for conventional processes, says Gaydon.
“Separate clarifiers and return sludge pumping stations are unnecessary. The concentrated biomass also substantially reduces tank volume and makes the plant footprint smaller,” he adds.
This lowers the direct plant costs for greenfield, brownfield, retrofit or capacity extension applications and existing treatment sites can often be used, rather than operators buying new land.
Further, owing to the reduction in mech-anical equipment, chemical-free operation and the high energy efficiency of the process, the operational and maintenance costs of a Nereda plant will be much lower than those of a conventional plant, DHV states.
Meanwhile, the nature of the technology also makes the plants easy to operate, Gaydon points out.
“Each Nereda plant is equipped with an AquaSuite Nereda Controller, which is a smart, integrated process controller. This ensures fully automated plant operation, reliable performance and ease of operation. It also enables unmanned or remote operation from a control room,” he explains.
The plant can be monitored from anywhere in the world and adjustments to the process condition of the operations can be made as soon as a complication has arisen.
The Wemmershoek WWTW, which will also use the Nereda technology, is currently under construction in the Western Cape.
Site establishment started about three months ago, with the digging of the foundations starting in May. The plant is expected to be operational by 2014.
The project entails the construction of an upgraded five-million-litre-a-day sewage treatment works, a new main transfer sewer pipeline from Franschoek to Wemmershoek and an outfall pipeline to the Berg river. Treated effluent will be pumped back to Franschoek through a separate rising main to irrigate parks and sports fields.
The capital cost of the five-million-litre-a-day Nereda plant will be about R40-million, which is significantly lower than that of conventional plants.
The Wemmershoek plant will also feature a Nereda AquaSuite intelligent controller similar to the one installed at the Gansbaai WWTW.
“We decided to go with Nereda for Wemmershoek owing to its small footprint compared with conventional systems, as well as the significantly lower electricity consumption,” says Stellenbosch municipality head of water and wastewater treatment Brett Keyser.
By making use of the Nereda technology, the municipality expects to save significantly on operational costs.
Further, this technology does not make use of primary and secondary settling tanks, which eliminates odour problems. The type of sludge produced is also easy to handle.
“Even though the quality of water produced by Nereda isn’t completely up to special standard at this stage, it can certainly still be achieved.
“The municipality does not believe that it is too much of a risk to make use of Nereda as the concept of granular sludge is well known and the plant can be automated,” says Keyser.