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Don’t let the FOG get you down

14 November 2016

Suzanne Gill reports on the benefits of dissolved air flotation technology, which can help food companies ensure that they remain environmentally responsible even when things are getting very foggy!

According to the result of research undertaken on behalf of Siltbuster Process Solutions (SPS), one-third of food and drink manufacturers say that effluent treatment will be a significant focus for investment in the coming year. 

Companies such as Dartmouth Foods are looking for ways to ensure that they meet their environment responsibilities. Two of Dartmouth Foods factories are responsible for producing shredded duck and chicken resulting in large amounts of fats, oils and greases (FOG) which was a strain on the local sewerage infrastructure. The company originally thought it had solved the problem with the installation of a grease interceptor, but it soon became clear that this solution was not able to cope, as Phil Maishman from Dartmouth Foods explains: “The grease trap very quickly filled up. It couldn’t keep pace with our production. We had to keep emptying it to stay compliant with the water company, but that was time intensive and we quickly knew that this would not be a suitable long-term, solution for us.”

The company turned to SPS who took samples of the wastewater and recommended a combination of a mix tank plus a packaged lamella Dissolved Air Flotation unit (DAF). 

Dartmouth Foods hired a DAF unit to test its suitability. Treating 1.5m3/hr, it neutralised the effluent, before the FOG, TSS and associated COD were removed. The removed fat was pumped to the mixing tank where it was mixed with other sludges from the site before being pumped into an anaerobic digestion (AD) facility. Given the success of the rented equipment, Dartmouth Foods went on to purchase its own DAF unit.

“Six months on we are incredibly pleased with our choice,” said Maishman. “The level at which the DAF unit is performing means we are now easily maintaining consent compliance, which is critical. We are keen to put our waste to good use in our own AD facility to create energy which makes our production even more cost-efficient and green.” 

How DAFs work
DAF units are commonly used for the removal of fats, oils and greases, suspended solids and associated Biochemical Oxygen Demand (BOD), which typically has poor settling characteristics. DAF units create ‘white-water’ by dissolving air under pressure into recirculated water – as the pressure is released when the water is returned back into the DAF unit, micro-fine air bubbles are formed, which attach themselves onto the solids, and rise to the surface. Here, the accumulating solids thicken and dewater before being removed by a mechanical scraper. The addition of lamella plates to a DAF allows higher flow rates to be treated by a compact unit.  A typical SPS customer, for example can utilise a 2.4m wide x 6m long unit to treat a peak flow rate of up to 144m3/hr. 

Modular DAF units can be combined in building-block fashion with other equipment to create tailored solutions, where ancillary mixed reaction tanks, containerised dosing and pumping equipment can be deployed along with the DAF to provide additional treatment capacity when it is needed. 

No spare capacity
SPS’s reasearch also found that 21% of food producers currently have no additional treatment capacity. Yet in the food and drink industry, particularly for producers of seasonal products, there can be significant production spikes which can create waste treatment issues – and therefore, cost implications. In response, many companies, such as Heineken’s Universal Beverages Ledbury (UBL) need temporary DAF equipment during busy periods. Indeed, UBL now regularly hires a bespoke large-scale temporary DAF system to allow it to treat peak effluent flows during the annual apple milling period. 

The dedicated fruit and vegetable milling facility is capable of processing up to 15,000 tonnes per day.  The apple milling period, which typically extends from late August to late November, pushes production to its peak, resulting in significantly higher levels of waste water with additional suspended solids and COD loading onto the site’s own large effluent treatment plant. 

At other times of the year, the site’s existing effluent treatment plant, which employs a combination of aerobic and anaerobic processes, is capable of treating the flow and load generated from the site’s production activities.   However, during the apple milling period, as the flow and load increases, existing treatment processes need to be supplemented. 

UBL first approached SPS Siltbuster Process Solutions in 2010 to address the problem and to provide temporary plant to protect the anaerobic reactor from the impact of the suspended solids. Its now regular, solution involves two packaged lamella DAF units and all the ancillary mixed reaction tanks, containerised dosing and pumping equipment required to provide the necessary pre-treatment capacity when needed. 

The SPS study also found that over 30% of companies have had an emergency effluent treatment problem in the past year which put their compliance at risk. DAF units can offer a solution in such scenarios as they can be quickly and easily installed on site. If necessary, they can be delivered by lorry-mounted crane and can be fully operational within hours of arrival. 
The proliferation of AD facilities in the UK has also created demand for feedstock, resulting in more competitive options being available for DAF sludge disposal. This can make a difference when evaluating the cost / benefit of using DAFs for pre-treatment to reduce Mogden charges or where investment to meet environmental obligations is required. 

Dartmouth Foods was just one company who took this route. While its DAF treatment plant was primarily designed to ensure discharge compliance it also has the secondary benefit of generating feedstock for the company’s AD facilities. 

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