MechChem Africa September-October 2022

⎪ Water, wastewater and pumping solutions ⎪

Left: WEC Model R plants are much smaller than conventional municipal rectangular plants. They can accommodate volumes from 250 to 3 000 m³/day and parallel modules can be added for higher volumes. Right: Trickle filters, which can be built using standard containers stacked on top of one another, are a very economical option because they have very few moving parts.

order to maintain these temperatures, the plant for the mine is being assembled within a larger enclosed facility,” explains de Koning. “The activated sludge biological treatment used in our Model A to D and Model R ranges of modular plants, is just one of our treatment solutions, though,” continuesGunter Rencken. “We have several other biological wastewater treatment technologies and configurations to suit a wide range of needs and capacities, depending on the applica tion, environment and other factors.We are also looking at innovative treatment methods to reduce reactor size, and at more advanced treatment methods. “As well as the larger Model R solution touched on above, we offer a trickle filter solution, which we can containerise for smaller flows; and we also do membrane bioreactors within containers, as well as mixed-bed bioreactors,” he says, adding that bioreactor technology is one of WEC Projects’ big focus areas in domestic and industrial wastewater treatment. The WEC Model R reactor is a semi-modular larger version of the Model A toDsolution. TheModel R consists of two concentric circular walls, with an inner tank as the clarifier and the space between the outer wall and the clarifier forming an annular aeration basin sized for the required retention and contact time. “The tanks and the plate components for the Model R reactor are all manufactured in our WEC factory or by external subcontractors. Then we basically ship them to site and assemble them on a circular swivel base,” de Koning explains. In the Model R CAS solution, wastewater from a sump is first pumped into an anaerobic zone, where phosphates are removed and Consumed Oxygen Demand (COD) is reduced. From the anaerobic zone, the wastewater flows into an anoxic zone for the removal of nitrates. The water then passes into the aeration basin for aerobic treatment,whereafine-bubblediffusedair systemprovides theoxygen required for the microbial growth. The biodegradable contaminants are ‘consumed’ and ammonia is converted to nitrates. After a required reaction time, theproduct ladenwater ispassed into theclarifier,where the remaining waste coagulates and settles to bottom of the tank as sludge. This is then extracted via a desludge valve and pumped onto purpose-built draining and solar drying beds or pumped tomechanical dewatering devices. “We are simulating the University of Cape Town’s biological treat ment process – the Biological Nutrient Removal Activated Sludge (BNRAS) system – in which ammonia, nitrogen and phosphorus are first removed by biological means at low costs with less waste sludge production. Through this process, we can achieve general discharge water quality standards for municipal wastewater treatment using a purely biological treatment method. We do sometimes have to en hance the process using chemical dosing, however, mostly to ensure

phosphate elimination, which is themost challengingof contaminants,” adds Rencken. “The footprint of theWECModel Rplant ismuch smaller than con ventionalmunicipal rectangular plants generallyused.We incorporate treatment stages into one single circular plant that can accommodate volumes from 250 to 3 000 m³/day – and we can also add parallel modules tomeet the needs for higher volumes,” De Koning continues. “Up inAfrica, many of our clients donot needdenitrification so they prefer ourmuch simpler tricklefilter systems, whichare another of our specialist packaged treatment technologies. Trickle filters, which can be built using standard containers stacked on top of one another, are a very simpleandeconomical optionwhen it comes tooperational expen diture, because they have very fewmoving parts,” De Koning explains. “They have an above- or below-ground septic tank and a recircula tion sump with two pumps. The containers are filled with a fixed-film designedmedia that looks like an array of funnels. The funnels slant at an angle to provide maximum aeration efficiency and oxygen transfer in the smallest volume. These allow the wastewater being sprayed onto the media from the top to flow down evenly, while air for aera tion is drawn up through natural convection from the bottom. The big advantage is that forced mechanical aeration is unnecessary, which simplifies the whole operation while saving on energy use. The rising oxygen in the counter current air transfers to the wastewater as it trickles down the media. Describing the biological reaction, Rencken notes that a biofilm containing mainly aerobic microorganisms is allowed to form on top of the filter media. This biofilm builds up naturally as the wastewater flows over the medium. Once formed, organiccontaminants inthewastewateraredegraded by the aerobicmicroorganisms in the outer layer of the slime, and am monia is converted to nitrates. The layer thickens through microbial growthandas thebiological filmcontinues togrow, themicroorganisms on the media surface lose their ability to cling on, causing pieces of the biofilm to fall off the filter. These are picked up by the underdrain system and transported to a clarifier for removal as sludge. “For higher flow volumes, we can we use a steel plate construction instead of installing the trickle filter in containers, or we can build a concrete structure,” de Koning says, pointing to an image of a much bigger systemwith a 500 m 3 /day capacity. “Africa, including SouthAfrica, presentsmany challenges forwater and wastewater treatment. Decentralised packaged solutions can bypass the need for massive investment in piping, pumping, reservoir and instrumentation infrastructure, making them ideal for many under-resourced, hard to reach or cash strapped situations,” Rencken concludes. www.wecprojects.com

September-October 2022 • MechChem Africa ¦ 13