Dissolved Air Filtration, United Kingdom
Engineering consultancy, WS Atkins was commissioned to provide solutions to Birds Eye Walls in order that they could reduce the costs associated with discharging wastewater into Anglian Water's sewer by the installation of local treatment plants.
The fishing town of Lowestoft can be found on the Suffolk coastline in east England.
CURRENT WASTE TREATMENT PROCESS
Birds Eye Wall's Ltd's Lowestoft factory produces a number of different food products, some of which are fried and therefore discharge oils and fats into the wastewater streams. The existing treatment facilities remove large product pieces and separate out some of the oils and fats prior to discharge into the North Sea.
At present there is no further treatment of the flows, but this has had to be reconsidered by Anglian Water due to the Urban Wastewater Directive. Therefore, future treatment may require preliminary and possible secondary treatment.
WS Atkins originally identified that a dissolved air flotation (DAF) plant would be beneficial for two of the production buildings.
The engineers were commissioned to complete a review of an internal environmental report followed by initial site investigations, provide confirmation of volume and character of wastewaters, arrange pilot plant trials for both dissolved air flotation (DAF) and ultrafiltration (UF) process technologies and provide conceptual design, budget cost estimation, specification generation and contractor selection.
The proposed process technology was designed to reduce total oil and fat content of 4,000mg/l to below 400mg/l with a flowrate of 10m³/h. Buffer storage of the feed wastewater was also specified by the provision of heated and insulated storage tanks.
DISSOLVED AIR FLOTATION (DAF)
DAF is the process of removing suspended solids, oils and other contaminants via the use of air bubble flotation. Air is dissolved into water, mixed with the waste stream and released from solution while in intimate contact with the contaminants. Air bubbles form, attach to the solids, increase their buoyancy and float the solids to the water's surface where they are mechanically skimmed and removed from the tank. A percentage of the clean effluent is recycled and super-saturated with air, mixed with the wastewater influent and injected into the DAF separation chamber. Air is injected under pressure into a recycle stream of clarified DAF effluent. This recycle stream is then combined and mixed with incoming wastewater in an internal contact chamber where dissolved air comes out of solution in the form of very fine bubbles that attach to the contaminants. The bubbles and the contaminants rise to the surface and form a floating bed of material that is removed by a surface skimmer into an internal hopper for further handling. This handling may include a belt filter press or a rotary vacuum drum with DE applied.
Dissolved air flotation systems are designed to remove fats, oils and grease (FOG), suspended solids (TSS), biological oxygen demand (BOD), food/animal production/processing wastes, industrial wastes, hydrocarbon oils/emulsions and many other contaminants. Clarification rates as high as 97% or more can be achieved using these systems.
CHEMICAL PRE-TREATMENT IN DAF SYSTEMS
Chemical pre-treatment can often help to improve the performance of contaminant removal.
Conventional DAF saturation design uses a recycle pump combined with a saturation vessel and air compressor to dissolve air into the water. This type of system, while effective, is expensive, labour intensive and can destabilize its point of equilibrium, creating burps due to incorrect, loss or creeping of EQ set-point in the saturation vessel.
DAF sizing takes into consideration many criteria for sizing including flow rate, water temperature, waste characteristics, chemical pre-treatment, solids loading, hydraulic loading and air to solids ratio.
DAFs are designed on the basis of the peak flow rate expected. The flow can range from 1 to 5 gallons per minute per square foot of surface area. Bench testing of waste stream samples is usually the preferred starting point when sizing equipment and determining proper chemical processes prior to the DAF. The chemical pre-treatment will assist and improve the DAF separation process.
Chemical pre-treatment often improves DAF solids removal efficiencies. The use of chemical flocculants with DAF is based on system efficiency, application and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeCI2 or FeSO4 or aluminium, such as AISO4. Organic and inorganic polymers (cationic or anionic) are often used to enhance the DAF process.
The wastewater pH may need to be adjusted between 4.5 and 5.5 for the ferric compounds or between 5.5 and 6.5 for the aluminium compounds using an acid such as H2SO4 or a base such as NaOH. In many applications, the DAF effluent requires pH adjustment utilizing a base such as NaOH to assure the DAF effluent pH is within the limits specified by the POTW (6-9 typically).
Attachment of most of the bubbles to solid particles can be effected through surface energies while others are trapped by the solids or by hydrous oxide flocs as the floc spreads out in the water column. Colloidal solids are normally too small to allow formation of sufficient air-particle bonding. They must first be coagulated by a chemical such as the aluminium or iron compounds mentioned above and then absorbed by the hydrous metal oxide floc generated by these compounds. Frequently, a coagulant aid is required in combination with the flocculant to agglomerate the hydrous oxide floc, increase particle size and improve the rate of flotation. Mechanical/chemical emulsions can also be broken through pH and polymer reactions.
DAF float often contains 2% to l0% solids. The solids may need to be dewatered before disposal to reduce the sludge volume by reducing water content.