Dr. Gunter Rencken and Rudolf de Koning of WEC Projects recently spoke to Peter Middleton, editor of MechChem Africa, about distributed wastewater treatment using packaged plants that can be purpose-customised and factory-built for easy shipping to and assembly on remote sites all over Africa.
WEC Projects, the South African EPC contractor specialising in water and wastewater treatment solutions, is about to complete the installation of a new WEC Model B packaged sewage treatment plant at the Mothae Diamond Mine in the Maluti Mountains of Lesotho. “The mine lacked a sustainable sewage solution for the 60 m3 per day of domestic raw sewage it was producing,” says Rudolf de Koning, sales representative of WEC Projects.
“The mine used sewage trucks, also known as honeysuckers to dispose of the sewage at the nearest disposal facility almost 130 km away. This was obviously a very costly method of dealing with locally produced wastewater. So we proposed the installation of one of our wastewater treatment solutions, our Model B conventional activated sludge (CAS) treatment plant with a daily capacity of 80 m3, which can accommodate future upgrading and expansions of the mine’s operations.”
The WEC Model B CAS plant is part of WEC’s modular range of sewage treatment plants that range from a Model A reactor up to a Model D reactor which consists of four reactors in parallel. “Within each reactor, there are anoxic and aerobic stages, along with a clarification tank. Each plant works as a complete wastewater treatment module, taking raw sewage in at one end and delivering discharge quality water out at the other, and the remaining sludge is pumped out to drying beds or mechanical dewatering devices,” explains de Koning.
A single reactor WEC Model A packaged plant can recycle 60 m3 of wastewater per day, with every additional module adding a further 60 m3/day of sewage treatment capacity. “So a Model B packaged plant like the one we are currently installing at the Mothae Diamond Mine consists of two reactor modules that can treat up to 120m3/day if the need arises. And we can put up to four packaged units in parallel to get a maximum capacity of 240 m3/h from this modular packaged solution,” he says.
Gunter Rencken, WEC Projects’ technical director continues: “We’re talking here about a relatively small volume per day of domestic wastewater treatment. In the flow range starting at 50-60 m3/day, one reactor can meet the needs of about 300 to 600 people, depending on how much wastewater each household in a community produces.
“When it comes to greater volumes, though, it becomes a question of economics. For requirements greater than the 240 m3/day a Model D packaged plant with additional reactors no longer makes economic sense, because one Model R self-contained packaged plant – which is built using a circular construction – offers cost and space savings compared to more than four rectangular modules,” he tells MechChem Africa.
De Koning says that a key advantage of the packaged approach is that units can be manufactured and pre-assembled at WEC’s local premises, where they can be properly configured and tested before being disassembled, loaded onto trucks and shipped to site. So the units are easy to transport and the installation time is significantly reduced, as is the overall footprint of the plants. “Model A to D and Model R plants are usually above ground and require very few civil works. Each plant simply requires a small pump sump or a buffer tank to absorb any shock flow, and a few concrete slabs for the reactor modules to sit on. The blowers used for the aeration zone, dosing units and MCC panels are all housed in a container,” de Koning tells MechChem Africa.
“This makes them ideal for mining camps like the Mothae Diamond Mine, as well for smaller municipalities – in South Africa and across the sub-Saharan African region,” Rencken adds.
The modular Model B plant at Mothae also integrates a WEC Wastemaster, which screens, de-grits and removes any oils, fats and non-biodegradable material upstream of the treatment process. “Besides the high fat and oil levels of this wastewater, the high altitude plant is also susceptible to low and fluctuating temperatures. Biological population growth occurs optimally at around 19 °C and above. In 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,” continues Gunter Rencken. “We have several other biological wastewater treatment technologies and configurations to suit a wide range of needs and capacities, depending on the application, environment and other factors. We are also looking at innovative treatment methods to reduce reactor size, as well as more advanced treatment methods.
“As well as the larger Model R solution touched on above, we also offer a trickle filter solution, which we can containerise for smaller flows; and we also do membrane bioreactors within containers, as well as Moving Bed Biofilm Reactors,” 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 to D solution. The Model 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 sludge retention time and hydraulic 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 Chemical 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, where a fine-bubble diffused air system provides the oxygen required for the microbial growth. The biodegradable contaminants are ‘consumed’ and ammonia is converted to nitrates. After a required reaction time, the product laden water is passed into the clarifier, where the remaining biosolids settles to the 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 to mechanical dewatering devices.
“We are simulating the University of Cape Town’s biological treatment 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 standards for municipal wastewater treatment using a purely biological treatment method. We do sometimes have to enhance the process using chemical dosing, however, mostly to ensure phosphate elimination, which is the most challenging of contaminants,” adds Rencken.
“The footprint of the WEC Model R plant is much smaller than conventional municipal rectangular plants generally used. 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 to meet the needs for higher volumes,” De Koning continues.
“Up in Africa, many of our clients do not need denitrification, so they prefer our much simpler trickle filter systems, which are 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 simple and economical option when it comes to operational expenditure, because they have few moving parts,” De Koning explains.
“They have an above or below ground septic tank and a recirculation sump with two pumps. The containers are filled with a fixed-film structured media 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 aeration 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 oxygen in the counter-current air flow 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, organic contaminants in the wastewater are degraded by the aerobic microorganisms in the outer layer of the slime and ammonia is converted to nitrates. The layer thickens through microbial growth as the biological film continues to grow, the microorganisms 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.
“Africa, including South Africa, presents many challenges for water 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.