A drain on resources

Effluent, grey water, dirty water - call it anything you like, except "waste water". According to Roger Clark, process and engineering manager at Waterlink, extending or replacing your effluent plant with one or more of an increasing number of treatment options could dramatically reduce your water bills, and minimise environmental risk.

Your effluent treatment plant might be looking a little tired, but with plenty of love and care it might still be doing a fairly adequate job in screening, flow balancing and pH correction. Your effluent treatment plant might be in quite good shape, thank you very much, but if the cost of buying in clean water and disposing of your processed water is still a disproportionately large overhead, then highly efficient and cost-effective technologies can now redress the balance. And the good news is that the payback time involved in upgrading your effluent treatment plant is shorter than ever. Used water that you once paid your local water company a fortune to dispose of can now be treated and recycled on site, and in some cases sold on to other users.

Treating just part of your plant’s effluent can provide water of a suitable condition for a variety of non-essential processes, including floor washing, vessel cleaning and irrigation. If you are still using tap water to clean the company’s cars and lorries you really do have money to pour down the drain.

The thought of re-using so-called wastewater to, say, wash dirt from vegetables, does not sound appetising to some production managers, but in numerous food processes suitably treated effluent is a perfectly harmless and far more financially viable source than using almost endless supplies of costly drinking water straight from the tap.

One step beyond

One of the first options to consider could be a clarifier such as a Johnson Lamella Separator which, compared to a settling pond, reduces space by as much as 90%. Often used for washwater recirculation units in potato and root crop processing, a clarifier uses a flow control system to prevent fouling. Models have also been developed for more specialised applications such as thickening and recirculation of sludge in anaerobic biogas processes in the sugar industry.

By applying modern controls to pre-war technology, one of the oldest wastewater treatment processes, the SBR (Sequencing Batch Reactor) is making a comeback. Based on an activated sludge process of nearly a century ago, the SBR is a front runner as a biological process. For high strength food processing wastes, biological processes can also feature jet aeration, but for low or slow-flow applications the SBR has a distinct control advantage because everything is in one tank.

Even though it is not so economical for highly variable flows, the end result is an effluent which consistently meets regulatory requirements for appropriate discharge. It is also a natural, uncomplicated process which can operate without the use of chemicals.

Some of the most recent SBR units use a treatment cycle where the wastewater enters from the inlet collection tank into the reactor, which is already half full of mixed liquor from a previous cycle. Whilst the full reactor continues to be mixed and aerated, incoming wastewater is diverted to a second reactor or stored. This react-only stage provides time to further bring the effluent to required discharge limits.

Real energy savings come into play after the decant stage because the reactor is idle – waiting, intelligently and cost-effectively. Labour intensive it most certainly is not, requiring less than one hour per working week from an operator, yet through its sheer simplicity this batch method does provide far greater control. With significantly reduced running costs and large capital savings, the SBR could be back for good.

Advance notice

Technology has also moved on a pace with new, longer-lasting, low pressure membranes. This now affordable technology is highly suitable for reclamation of wastewaters, product recovery and production of high purity water. New trailer-mounted units can demonstrate the effectiveness of the membranes in removing dissolved solids and also allow different membranes to be evaluated against the required treated water quality. Data from trials can then be used to gain a complete picture of the performance and reliability of a full-scale membrane plant. The design of a system must carefully consider pre and post treatment in order to provide requisite quality and an acceptable membrane life. Reverse osmosis has often proven too costly to produce water due to the energy used by high pressure pumps. However, recent technological advances have significantly reduced membrane operating pressures whilst simultaneously increasing efficiency. A stage beyond membranes is an ion exchange mixed bed plant, which can produce the grade of ultra-pure water required by power stations and pharmaceutical companies.

For the final filtration of biologically treated wastewater, a sand filter, such as a Dynasand, can be added as a tertiary step to polish the water in order to help gain Environment Agency consent to discharge to sewer. This sand filter does not require flocculation tanks because it can handle high concentrations of suspended solids. It also has no pumps or automatic valves, and even the filter itself has no moving parts. Energy consumption is extremely low and chemical usage almost zero.

Depending of course on the size of your food processing operation, more than one of these options could be complete overkill, but don’t be put off by the thought of some mammoth extension to your effluent treatment system because the footprints of today’s plants are very small.

Whether you are producing food continuously or in batches, the technology can be tailored to suit your process. For such a valuable and flexible commodity, “wastewater” seems such an inappropriate description.

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