As water authorities tighten regulation and increase surcharges, the trend is for companies to treat and reduce their biological oxygen demand (BOD) load onsite. BOD is a measure of the amount of oxygen required by a biological process to consume the amount of organic matter in a waterway. If all of the oxygen is consumed, natural plant and animal life dies off. For any food processing company, managing BOD can represent a costly challenge.
Food processing and production often is a water-intensive process, which can generate significant quantities of high-strength wastewater. Discharges typically contain elevated levels of BOD and total suspended solids (TSS). Due to the strength of these effluents, many food and beverage producers regularly contend with local water-authority surcharges, tipping and waste-hauling fees.
Food manufacturers use water for general cleaning, rinsing, batching, blending, diluting and chemical cleaning in place (CIP). Wastewater also can come from unwanted liquid batches, spills, tank overflows or discarded liquid packages.
A multinational confectionery producer with an established manufacturing facility in Australia was facing increasing discharge fees from the local government for its trade waste liquid discharge. As part of the company’s effluent improvement program, they sought to reduce the BOD load in their discharge wastewater to support environmental commitments and reduce discharge fees.
The local regulating authority requires all businesses discharging wastewater to comply with the New South Wales (NSW) Liquid Trade Waste Sewer Guideline Limits. Acceptance parameters taken from the guideline are shown in Figure 4, page 15.
The guideline applies a non-compliance excess mass charging formula with an exponential term that includes BOD5 concentration. For this customer, the annual discharge fees could have been greater than $95,000 without onsite treatment.
Ease of operation and maintenance, as well as the ability to meet varying loads, were key considerations for the client. A maximum two-year payback period was also part of the client’s selection criteria. As a proof of concept, a demonstration trial also was required onsite.
Biological Treatment Technology
BioGill technology is an above-ground non-submerged bioreactor providing attached growth planar media with a nanoscale ceramic structure suitable for microorganism attachment. The media is configured to provide two distinct regions: an air side and a liquid side. This is done by folding the media sheet over a top hanger bar to creat an inside air pocket. The space inside is called the air side, and the space outside, between two adjacent sheets, is called the liquid side. A bank of sheets together is called a “BioGill” and a single sheet is called a “gill.” Wastewater is fed from a specially designed, solids- and fouling-tolerant distributor located at the top, and flows downwards by gravity through the liquid side (See Figure 1, page 14).
Plant Design & Installation
Following a successful onsite pilot project, the client awarded a contract to design, build and commission a full-scale effluent treatment plant. The plant, commissioned in January 2015, was designed according to parameters in Figure 4.
The plant included a 60-cu meter (16,000 gal) balance tank with a mixer and a transfer pump; a 30-cu meter (8,000 gal) recirculation tank with recirculation and discharge pumps; nine BioGill bioreactors housed inside thermally insulated cool room sandwich panels; pH correction systems in the balance and recirculation tanks; a chamber ventilation fan with temperature control; urea and phosphoric acid timer-based dosing systems; and a hot water solar heating system (See Figure 3, page 16). The plant was automated using a PLC. All analog signals to the PLC, including flows, temperature and pH, were data logged.
Inground concrete tanks were used for better temperature retention and to simplify the hydraulics because BioGill bioreactors rely on gravity drain. The balance tank was sized to hold at least two full days’ influent volume. When the recirculation tank cycle is finished, the tank is emptied, and a batch is transferred from the balance tank to the recirculation tank.
Since water pumping energy is the only energy for the system, as opposed to plants using air blowers, the BioGill system energy management was considered in the design. The recirculation pump pressure losses included pipe losses (3 meters), static head over BioGill (3 meters), BioGill feed manifold (1 meter), with total head approximately 7 meters, plus a few meters of extra spare head for future expansion (a 3-meter head heat exchanger for solar heating was added later).
The collection bund underneath the bioreactors was built with a gently sloping floor to prevent sloughed off sludge from flowing to the treatment tank. The higher end of the floor is where the overflow pipe to the inground recirculation tank was located.
BioGill media does not require any chemical addition as they mainly function as an attached growth media. The chemicals required are associated with the overall process and not the BioGill component. Chemicals were used for pH correction and nutrient addition. Healthy biomass requires a feed pH between 6.5 and 8.5. The gills themselves are resistant to short exposure of high pH up to 10 or low pH down to 5 for a few hours.
No coagulation chemicals were used for solids capture. The bulk of solids slough off the media and fall down to the bund underneath. Suspended solids in the effluent were within the 300 mg/L due to biomass being attached to the media and not suspended in the water. BioGill recommends C:N:P ratio of 100:10:1. After commissioning, the amount of chemicals dosed was reduced to 50% of the stoichiometric ratio.
The process hydraulic residence time varied according to influent strength, with COD reduction typically between 65% and 95%. Sampling of influent and effluent COD and percent of COD removal showed a wide variability of the influent with corresponding discharge COD. The client aimed for 900 COD mg/L or less to meet the three times 300 mg/L BOD requirement as specified by the NSW liquid trade waste.
After start-up, a biomass becomes established on both sides of the attached growth media. The bioreactors provide a high surface area, flat sheet media for biomass acclimatisation to the available substrate. The flat sheet media accommodate both aerobic and anaerobic bacteria depending on the side of the media.
The media has a first biolayer on the air side of the media comprising aerobic cells or microorganisms and a second biolayer on the nutrient/water face of the media biolayer comprising anaerobic cells. Over time, biomass sloughs off into the collection bund.
Biofilms found in the gills are made of many layers of microorganisms varying both horizontally and vertically. The microorganisms at the top may differ from the microorganisms at the bottom. Those deep inside the biofilm may differ from the ones growing on the edge. For example, in a dairy installation using BioGill technology, fungi were observed growing on the outside airspace layer.
The full-scale treatment plant, using this attached-growth bioreactor technology, has been running for more than two years at this food manufacturing facility. The technology was selected after documented piloting and subsequently met the client’s return on investment requirements.
The treatment performance results showed that this new type of bioreactor technology effectively treated high-strength COD wastewater onsite from an average feed concentration of 2,023 mg/L to a discharge concentration of less than 400 mg/L COD and 100 mg/L BOD.
This led to reductions in the liquid trade waste at the plant, full compliance with the regulatory requirements, and significant savings in discharge fees.