Reducing effluent costs with DAF
17 June 2010
Developed commercially in the ‘50s, dissolved air flotation (DAF) is a proven treatment process for wastewater. In today's economic conditions, more food processors are looking closely at cost-effective ways to reduce effluent costs. By ANDREW MILEY
The adopted processes for food industry wastewater are always dependent on the receiving body, be it the local sewage treatment plant, river or ocean discharge - or even recycling.
Food industry wastewater usually contains high levels of fat, BOD, solids and in some cases, nutrients. Treatment of food industry wastewater will be primary treatment only or a combination of primary and secondary treatment processes.
The preliminary treatment stage will nearly always involve some form of screening to remove gross solids and foreign material. Balancing or equalisation of the wastewater is essential in food processing applications owing to the highly variable wastewater characteristic observed during normal processing hours and cleaning.
A DAF system is generally used for fat and solids removal. If the dissolved air flotation process is followed by biological treatment then often chemical treatment of the wastewater isn’t necessary. However for sewer discharge some coagulation, pH correction and flocculation processes are normally used. With an appropriate chemical regime, a DAF system will remove more than 90% of fats and solids and up to 70% of the BOD load.
Operating costs of DAF systems are favourable compared to almost all other process technology - more so than in recent times with spiralling energy costs because a DAF's power and air consumption are low – also there is the added bonus of minimal maintenance.
The sludge-thickening nature of a DAF considerably reduces the volume of sludge. Dry solids concentrations in the sludge can reach 10% in certain applications compared to fractions of 1% for settled sludge.
As the DAF process is a continuous operation, overall capacities of the system are therefore largely dependent on hours of operation. The smallest commercially available DAF systems generally rate at a few thousand litres per hour, although factors such as solids loading are critical to final sizing. As factory-built units, DAF systems can handle in excess of 0.5ML per hour. Larger units are generally custom-designed and flotation tanks are constructed on site in concrete.
A common myth is that clarification is less expensive than DAF. While it’s true the mechanics of operation aren’t quite as involved as DAF, but space requirements, operating costs and performance make the DAF systems a clear winner.
Although this wastewater treatment technology comes in many shapes and sizes, the principles of operation are similar. Separation occurs by allowing air bubbles to attach to particles in a polluted stream, forcing them to the surface of a vessel, which is fitted with a scum removal mechanism.
The primary advantage of DAF over other forms of solid-liquid separation is that air bubble attachment reduces the specific gravity of particles that might normally sink, forcing them to float. This results in two distinct and easily separable phases - the treated water and surface sludge.
Over the past few decades, there have been considerable improvements made in DAF technology. Modern DAF systems are generally rectangular owing to the more compact sizing, modular expansion capabilities, better sludge removal dynamics and the ease of combining chemical reaction tanks into packaged designs. DAF systems are fitted with a surface sludge scraper, usually in the form of a chain driven blade assembly.
A current trend leans strongly towards scraping the sludge in a counter current direction that has proven to be highly beneficial in overall performance. DAF systems are also fitted with bottom (or settled sludge) removal systems when required.
While the principle of DAF is relatively simple, it should be understood DAF is basically a physical separation process and the performance of such equipment is strongly dependent on the nature of the incoming stream. It may be best to start with a pilot study.
Pre-treatment of effluent can be crucial to the effectiveness of the DAF process. A DAF system will only remove those particles that the air bubbles can attach to. If the pollutants are initially in an emulsion or a fine colloidal dispersion, to enable any reasonable degree of treatment coagulation and flocculation processes are required. A properly designed sludge removal system is also essential.
With all elements in place, DAF is one of the most reliable of all solids-liquids separation processes. They are usually fully automated requiring minimal operator attendance.
Andrew Miley is from Eimco Water Technologies, which has experience in the treatment of food processing wastewater, from meat and poultry to paste and rice, fruit and vegetable to tofu and related areas such as industrial or commercial kitchen wastewaters, prepared meals and flight catering facilities
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