Tackling difficult effluent: A guide

WPL Disolved Air Floatation (DAF) wastewater treatment tank.
WPL Disolved Air Floatation (DAF) wastewater treatment tank.
SAF treatment plants have a small footprint and are most suited to biodegradable waste.
SAF treatment plants have a small footprint and are most suited to biodegradable waste.
Industrial effluents are complex and companies should ensure that they do not breach consent rules as this could result in fines.
Industrial effluents are complex and companies should ensure that they do not breach consent rules as this could result in fines.
Fish blood, gut, scale and bones in a WPL Drum Screen.
Fish blood, gut, scale and bones in a WPL Drum Screen.

Difficult-to-treat effluent is often the result of challenging industrial processes. These processes can create complex wastewater streams which may have high levels of oil and grease; heavy metals; and high bacterial and viral loads that require specialist treatment. Additional challenges may come from a huge range of micro-pollutants. Hard-to-treat effluent may have high biological oxygen demand (BOD) or chemical oxygen demand (COD) as well as high levels of suspended solids. Treating industrial effluent is complex and depends very much on the kind of load and the nature of the process generating the wastewater stream. WPL, a wastewater treatment provider, has had to tackle many challenging effluent streams over the last 25 years.

Treatment variation

It is rare to find two industrial wastewater streams that are the same, so treatment will vary from site to site. Because WPL’s plants are designed for each job, the company has found a range of ways of dealing with difficult effluents. A fish farm in Scotland, where the effluent contained a high level of biological material, required a combination of submerged aeration filter (SAF) plants and dissolved aeration filter (DAF) plants, as well as a series of settlement tanks. At a blood-processing laboratory in England, engineers were able to customise the existing treatment works by fitting fine screens which could filter out troublesome lumps of biological matter. For a municipal waste disposal site in England, WPL created a chemical treatment which could remove the variety of pollutants seeping from black bin-bags.

Treatment options

Each case is different when it comes to the kind of treatment an industrial user requires.   SAF technology is most suited to biodegradable waste where there may also be high ammonia levels.  SAF treatment plants have a small footprint and are economical. If separating suspended solids is the main issue, then a DAF plant may be more effective.  DAF plants are often used in abattoirs, fish processing or other cases where there is a high level of particulate, biological solids. In some cases, for instance where there is a high concentration of hair, it may be important to fit screens. Additional chemical processes may be necessary to clear micro-pollutants, including heavy metals, bacteria and viruses.

Assessing effluent

Each case begins with a site visit in order for WPL engineers to understand the nature of the site and the nature of process taking place.  Initially it is important to assess the quantities of effluent being produced.  If the effluent strength is variable, this also has to be taken into account. Effluent samples are taken to WPL’s in-house laboratory and assessed for levels of COD, BOD and a range of micro-pollutants.  The samples are then subjected to a range of different processes to assess their treatability with biological, physical and chemical processes. Once it has been established that the effluent can be successfully treated in the lab, it is important to carry out a pilot study onsite.

Pilot study

During the pilot study, engineers will establish the parameters for treatment as well as calculating the whole-life running costs.  Treatment options can be adjusted during the pilot to ensure the effluent is treated to optimal standards for a given site. At a large site, the pilot can be carried out by intercepting a side stream of effluent, taking as little as 5% of the total produced onsite.  In other cases, the pilot study will treat the entire effluent stream. A pilot study is carried out for one week on a chemical process and eight to twelve weeks for a biological process. Both capital and operational costs can be calculated, taking into account energy use, chemical costs and maintenance.  

The benefits

Sewerage charges for industrial users are calculated by utilities using the Mogden formula. This means costs vary from month to month, depending on the strength of the treated effluent released into the public sewers. In some cases sewerage charges for industrial and agricultural customers can increase tenfold if an unusually high level of contaminants is detected in effluent streams. Industrial users can even face fines if they exceed consents set by the water utility. For these reasons an increasing number of industrial clients are looking for stable, reliable onsite treatment which treats effluent to a consistently high standard. It is important to find the right combination of treatment for the effluent being treated which is why WPL takes a systematic approach to the issue. Hundreds of thousands of pounds are wasted every year on industrial treatment plants which are inadequate or unsuited to the job in hand.

Investing in a custom-made plant, designed according to the nature of the effluent to be treated, is the answer to tackling hard-to-treat effluent.  It is more efficient, better for the environment and could deliver long-term cost savings. WPL has been called in to tackle some challenging effluents over its 25-year history. Here are a few of the worst:

Disposal site for zoo animals

An animal carcass disposal centre asked WPL to help it manage variable loadings from its rendering operation. The centre was responsible for disposing of the bodies of large animals such as whales, lions, elephants and dolphins on behalf of zoos and wildlife parks. The effluent produced by the site where the animal bodies were dismembered and rendered was of highly variable strength and load, depending on the kind of animal carcasses being processed. WPL installed a treatment system where the effluent was homogenised in a mixer tank then aerated and screened, prior to treatment in a SAF tank. It was then allowed to settle before being discharged. The untreated effluent had a COD of between 8,000 and 10,000 mg/l. After treatment COD levels were under 500mg/l.

Blood processing plant

A blood processing plant, which uses human blood to manufacture a range of blood-related products, was having difficulties managing high levels of biological solids in its effluent. WPL engineers assessed the existing treatment works and discovered that the settlement tanks were not working efficiently.  The issue was addressed by adding fine screens to the end of the process to separate out the biological solids which were being missed by the treatment plant. By identifying the cause of the problem and improving the process, WPL saved the client from the expense of replacing the whole plant.

Municipal waste

A solid waste company processing black bin-bags was having difficulty treating the leachate seeping out from the crushed domestic refuse. The variability of the waste meant that identifying the appropriate treatment combination in this case was particularly challenging. WPL carried out extensive testing on dozens of samples in order to develop a customised chemical treatment process specifically designed for black bin-bag ‘juice’.

Fish processing

A fish processing plant in Scotland needed help with its effluent which contained blood, scales and bones and was proving hard-to-treat. WPL designed a customised plant which fed the effluent through a filter before moving it to a DAF plant to separate out the solids. This was followed with treatment in a SAF plant to remove soluble pollutants. Contaminants were reduced by 95% and the treated wastewater achieved environmental compliance for release into the sea.