Reducing COD levels for seafish processing

Coastal fish processors face a dramatic increase in trade effluent disposal costs as a result of the implementation of EU urban wastewater directive. In response the Sea Fish Industry Authority (SFIA), a statutory body working for the UK fish industry, has developed simple, innovative technical solutions which businesses can use to keep the charges to a minimum.

To identify the scale of the problem the authority, also known as Seafish, carried out detailed water and effluent audits of twelve companies representative of the main processing sectors of the industry. Trials were also carried out to assess the effectiveness of various types of ‘end-of-pipe’ treatment equipment.

It became clear from the audit that for most companies, the cheapest and most effective way to reduce these charges was to tackle the problem at source, using waste minimisation techniques. Seafish Technology, the authority’s research arm, then concentrated on identifying changes to working practices and developing simple technical modifications to processing equipment to achieve savings. Two such areas were separator catch baskets and waste separator chutes.

Catch baskets are used in the drains of most processing areas to keep solid waste out of the effluent stream. In terms of reducing trade effluent charges the traditional perforated plate catch basket design is largely ineffective (fig. 1).

The drain covers often have large slots which allow large solids to enter the basket. The baskets are often poorly fitting, resulting in waste by-passing the basket. Conversely, if the baskets themselves have very small aperture holes they block quickly and the tendency is for the employees to permanently remove the baskets. Most importantly, the effluent washes through the waste trapped in the basket, increasing in strength as it picks up dissolved organic material and small suspended solids, in some cases the waste on the bottom of the basket remains soaking below the water level in the drain. Even pouring clean tap water through a typical catch basket containing white fish waste increased the Mogden-calculated trade effluent charge of the resulting effluent from £0.46/m³ to £1.52/m³, based on Yorkshire Water’s 1999 – 2000 charges.

To deal with these problems Seafish Technology has developed a new design of catch basket. The prototype separator catch basket shown in fig. 2 was built and tested in an existing point drain in a white fish processing area.

The drain cover keeps large solids out, whilst optimising the flow of the effluent onto a four-sided wedge wire screen with 1mm apertures. The solids slide down the wedge wire to be collected in the waste basket. The liquid passes through the wedge wire and is diverted away into the drain without passing through the solids. In trials comparing the new separator design to the traditional basket, average effluent strength (sCOD) was reduced by 53%. In real terms this resulted in an average 41% reduction in Mogden-calculated trade effluent charges from £1.99/m³ to £1.16/m³, based on Anglian Water’s 1999 – 2000 charges.

Dirty washing

This separator principle can also be applied with great effect to processing operations. Typically, the waste from processing machines is flushed down a chute into a catch basket or bin. Here too the effluent washes through the waste receptacle and increases in strength. By simply inserting a section of wedge-wire screen into the waste chute, the liquid can be prevented from washing through the solids, significantly reducing the strength and treatment cost of effluent.

A prototype chute developed for the Baader 51 skinning machine is shown in fig. 3. When tested it reduced effluent strength (sCOD) by 65% corresponding to a 49% reduction in Mogden-calculated trade effluent charges. Ongoing trials with a heading machine waste chute indicate a 75% reduction in strength may be achievable.

Seafish has produced Guidance for Fish Processors on Water and Effluent Minimisation to help businesses help themselves. Large reductions in water and effluent costs can be made by adopting simple measures. Technical development work continues and technical data sheets are to be issued.

Figure 1 Key:

• 1 Effluent
• 2 Drain cover with large slots
• 3 Perforated plate basket
• 4 Solid waste
• 5 Washed out small solids
• 6 High-strength, screened effluent
• 7 Effluent level

Figure 2 key:

• 1 Effluent
• 2 Drain cover
• 3 Wedge-wire separator chute
• 4 Screened effluent
• 5 Basket
• 6 Solid waste
• 7 Effluent level

Figure 3 key:

• 1 Skinning mechanism
• 2 Existing chute
• 3 Effluent
• 4 Solids
• 5 Longitudinally-running wedge wire
• 6 Effluent
• 7 Skins
• 8 Effluent
• 9 Skins