Tackling waste at sauce

ACWa Services has extended the effluent treatment facilities at Hazlewood Foods' Selby site following bottling plant expansion

Due to the expansion of business in cooking sauces and the completion of a new specialist dressings' bottling facility at its manufacturing centre in Selby, North Yorkshire, Hazlewood Sauces and Pickles is extending its effluent treatment plant to treat higher volumes of process wastewater. The design and build contract, which has been awarded to Skipton-based ACWa Services by plant operator United Utilities (UU), will allow Hazlewood to treat a projected 30% increase in effluent flow and load while operating well within the revised discharge consent.

Hazlewood Foods is one of Europe's leading manufacturers of own-brand convenience foods and a corporate member of Greencore. Recently, the increasing volume of process wastewater discharged from operations at the Selby factory put pressure on the existing effluent treatment facility and, due to limitations in hydraulic capacity, the plant was unable to satisfy changing process requirements. The existing plant, designed and installed by ACWa in the early 1990s, had been effective and trouble-free when operating within the limits of its intended design capacity - an instrumental factor in UUs' decision to select ACWa as main contractor.

getting in a pickle

When formulating the revised process design, ACWa took into account the fluctuation of volumetric throughput and the COD/BOD loadings produced by seasonal changes in production. Due to the pickling season, the levels of effluent have a tendency to increase during September-January and decrease during February-August when the plant switches to sauce production.

At the re-design stage it was also necessary to consider the strict requirements of volume and flow. Following an evaluation of the existing treatment process in relation to revised customer requirements, ACWa submitted detailed recommendations and updated design proposals, which would allow the system to handle an average flow of 2,500m3/d with a maximum flow-rate of 4,800m3/d. The process would also be capable of treating a maximum COD loading of 12,000kg/d with an effluent discharge quality of 750kg BOD/d. The contract for extending and upgrading the Selby plant comprises the design, supply, installation, testing and commissioning of all the mechanical and electrical systems, together with associated civil engineering works.

The new process will include effluent collection and transfer, screening, primary settlement and balancing, pH correction, nutrient dosing, heating, anaerobic treatment, sludge removal and storage, bio-filtration, final settlement and odour control.

At the Hazlewood site, wastewater gravitates to an existing transfer sump from which it will be pumped to a screening, primary settlement and balancing facility. Alternatively, if the effluent is of an unusual strength or the balancing/pre-acidification tank capacity is exceeded, it will be directed to a new 300m3 calamity tank, installed as an additional holding facility.

The calamity tank will allow effluent to be fed back into the system at times when there is a reduction of incoming flow or will hold it in readiness for tankering away from site. The new balancing and calamity tanks will be capable of handling any abnormal peaks in flow. A mixer will be installed to ensure complete homogenisation of the effluent and effective chemical mixing while creating optimum conditions for the downstream hybrid reactor process.

A natural process

The reactor process is achieved by allowing naturally occurring bacteria in the effluent to partially acidify the organic content of the COD. To address the problem of odour, the plant will incorporate an emission control system based on an ACWa AIR packed tower wet scrubbing process.

In operation, exhaust air streams from the balance and calamity tanks will be ducted to a local exhaust fan. This will be installed to provide the required exhaust suction and discharge pressures to compensate for pressure drops in the ductwork. Gases from the balance and calamity tanks, combined with exhaust gas discharged from the upstream bio-filter, will be ducted to the main exhaust air fan, which provides pressure to overcome the resistance in the ductwork, scrubber and stack.

Extractor hoods installed over the sludge tanks will also direct odours into the scrubbing system during the decant cycle. Effluent from the collection sump will be mainly pumped to the inlet of a new rotary screen, which will discharge screenings to a compactor for off-site disposal.

Screened effluent will then gravitate to a new primary settlement stage installed with a swirl-flo clarifier. At this stage of the process, duty/standby pumps will remove gross solids from the effluent and transfer them to a sludge storage facility and, under normal operating conditions, clarified liquor will gravitate back to the balancing stage. As it is necessary for effluent in the downstream anaerobic treatment process to have a pH of between six-eight, effluent in the balance tank will be corrected by dosing with sodium hydroxide.

This will be pumped from an existing storage tank, monitored and controlled by an ultrasonic level transmitter. The dosing pump will be controlled and activated by a pH monitor.

At this stage of the process, liquid nutrient will be dosed into the effluent to maintain the correct level of nitrogen. This also contains the micro-nutrients identified as being deficient in the effluent, preventing a condition that would retard biological growth. The nutrient solution will be dosed from a bunded bulk-storage tank by two pumps operating as duty/standby. Due to the downstream anaerobic treatment process operating at optimum efficiency at around 35-37°C, it is often necessary to raise effluent temperature.

For this purpose, two heat exchangers operating as duty/standby are to be installed on the outlet of the reactor to recover heat from the effluent before it undergoes further treatment in the biofilter. A second set of duty/standby heat exchangers will be installed on the inlet to the reactor.

These will be connected to hot water boilers and will ensure effluent is pre-heated to the correct temperature before entering the reactor. A temperature control system will be installed across the heat exchangers to ensure the correct temperature is maintained and hot water will be provided by a duel-fuel boiler utilising biogas produced in the anaerobic reactor. ACWa selected anaerobic filter technology for the Hazlewood treatment process because of its reliability and track record in the reduction of BOD/COD levels by at least 70%, while producing only small volumes of sludge. Effluent will be transferred to the anaerobic reactor inlet via a pipework distribution system. In operation, incoming effluent, composed of biologically formed granules (biomass), flows upwards through the sludge blanket.

Treatment occurs as the carbonaceous organic matter makes contact with the biomass. The gases produced under anaerobic conditions - mainly methane and carbon dioxide with traces of hydrogen, hydrogen sulphide and water vapour - cause internal mixing, which facilitates the formation and maintenance of the biomass. Gas bubbles produced within the sludge blanket remain attached to some of the granules, causing them to rise and strike the media. At the point when bubbles are released and de-gassed, granules fall back into the sludge blanket. A thin layer of anaerobic biomass forms on the media to facilitate the reduction of COD and anaerobically treated effluent is discharged, via a supernatant peripheral weir, into a degassing chamber for the removal of any entrained biogas. Biogas produced during the anaerobic process will rise to the top of the reactor and pass through a single outlet via a flowmeter to the dual fuel boilers or to a waste gas burner.

Thermo-wells installed in the reactor will monitor the temperature of the sludge blanket and the effluent. From the anaerobic digester, treated effluent is pumped into the existing bio-filter, which is being retained as part of the new process.

Within the biofilter, effluent will be distributed over the upper surface of the packing media by a reaction driven rotary distribution system. As effluent passes down through the media, biomass growing on its surface will adsorb the soluble organic matter, converting it to cellular material or the eventual breakdown products of carbon dioxide and water.

Excess cellular material will pass from the base of the filter into a re-circulation sump. To handle the increased volume of effluent passing to final settlement, a larger tank is to be installed to operate in parallel with the existing one.

The flow of effluent will be split between the two tanks via a splitter box. When completely settled, clarified effluent will overflow a peripheral v-notch weir and gravitate to a buffer tank before being transferred to the river via a discharge pump



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