Our expert guide to treating industrial effluent, often referred to as trade effluent, examines the use and selection of speciality wastewater treatment chemicals including coagulants and flocculants and how they work. It reviews common treatment techniques including clarification, dissolved air flotation (DAF), filtration and biological treatments. It also considers techniques used for sludge dewatering and concludes with a guide to selecting the best treatment products for various wastewater streams.
Tougher environmental standards
With the introduction of ever tougher environmental standards, selecting the best treatment processes for a business’s industrial wastewater and effluent is becoming increasingly important.
For many businesses going beyond simply meeting the discharge consent standards set by their local sewerage undertaker is becoming more important. Treatment costs, opportunities for process optimisation and energy efficiencies are also becoming increasingly significant, together with opportunities for more environmentally-friendly water reuse solutions.
How is trade effluent defined?
In the UK any wastewater that is produced as part of an industrial or commercial process is referred to as trade effluent. The UK’s Water Industry Act 1991 describes trade effluent as any liquid, either with or without particles of matter in suspension in the liquid, which is wholly or partly produced in the course of any trade or industry carried on at a trade premises.
This means that if your effluent stream is not simply uncontaminated rainwater or domestic sewage then you need to treat it differently before discharging it.
Treating industrial effluent using chemicals
The main chemical products used in the treatment of industrial effluent fall into two categories:
The need to use either or both coagulants or flocculants will depend on the nature of the substrate being treated.
The size of the particles will determine whether the substrate is stable or unstable.
If the substrate is very stable then it will likely comprise highly colloidal particles with a high charge density or may even be partially or totally soluble. In this case it will be necessary to use a coagulant.
The process of coagulation
Particles within the waste stream are kept apart by Brownian motion and similar charged surfaces will repel each other.
The main charge on colloidal particles is typically negative and hence, it is the addition of a cationic coagulant that will neutralises this negative charge.
Once charge neutralisation has occurred then weak forces of attraction can predominate and the particles can come together to form larger particles or micro flocs which is an important part of the treatment process.
Types of coagulants used to treat industrial effluent
There are two primary types of coagulants commonly used to treat industrial effluent streams and they are as follows:
Inorganic coagulants are the most commonly used wastewater treatment coagulant having highly charged metal ions. These coagulate by charge neutralisation and also form hydroxide products that encapsulate the destabilised effluent particles.
Inorganic coagulants are relatively cost effective and are therefore sometimes the only cost effective way of chemically treating the effluent; chemical oxygen denand (COD) for instance.
However, the hydroxide sludge produced is often very bulky and does not compact readily.
Inorganic coagulants are very acidic and their use can result in the need for pH correction to maintain either a viable treatment (the metal salt may become soluble at low pH taking the treated effluent back into solution) or meet the discharge consents set for pH.
Dosage rates for inorganic coagulants in the order of a few hundred to a few thousand ppm are not uncommon.
Organic coagulants are very low molecular weight, highly cationic charged polymers that act to destabilise the substrate by charge neutralisation and possibly a bit of bridging.
They do not form hydroxide flocs and therefore generate less sludge.
They have negligible effect on the pH of the effluent.
Dosage rates in the order of a few tens to a few hundred ppm are not uncommon.
Combined organic and inorganic coagulants
The third treatment approach is to use both organic and inorganic coagulants either separately or as a blended treatment product. This approach helps to combine the benefits of both types of coagulant and can be very useful under certain circumstances.
Flocculants used to treat industrial wastewater tend to be low charge (compared to coagulants) high molecular weight polymers. They come with three charges:
Flocculants come in three physical forms which are:
Having destabilised the effluent the settlement rate may still not be adequate. If this occurs then it may be necessary to apply a flocculant to improve the settlement rates.
How do flocculants work?
Flocculants work by bridging the micro flocs together. The charged sites on the flocculant molecule attach to the micro flocs, but because of the length of the flocculant molecule, different charge sites on the same molecule can attach to different micro flocs. These micro flocs are then chained together to form a larger floc. The creation of larger flocs helps to improve the rate of settlement.
It is important to remember that for a flocculant to work effectively, enough time must have been allowed for the coagulation process to complete.
Flocculants are also used to aid dewatering of the sludge produced after settling of the effluent.
Techniques for effluent treatment
Common techniques used in the industrial effluent treatment process include:
Clarification usually refers to the settlement of the effluent in either in a pit or a purpose built settlement chamber. The treated effluent weirs over from the top and the settled solids are pumped from the bottom.
Increasing the floc size tends to increase settlement rates but can also cause poor fines capture. Typically operators should aim to create a floc of between 1.5-2.25mm.
Usually sand filters where the industrial effluent or more commonly water is treated and then passed into a filter to remove the solids. Filtration requires small flocs 1.0-1.5 mm to give good filter penetration. Too much polymer will give a large floc that will filter out on the surface of the sand blinding the filter material more quickly. Also excess polymer can make backwashing of the filter more difficult resulting in filter cracking and mud balling.
Biological treatment using activated sludge plants or bio-filters are used when the effluent requires further treatment or it is not possible to remove the pollutant effectively by chemical treatment – for example – the treatment of sugars and starches.
There is a negligible requirement for the use of treatment chemicals in the process if the biological plant is not overloaded. However, if the sludge is bulking at the settlement stage, the addition of a wastewater polymer can sometimes be used at a couple of ppm.
Additionally, some effluent treatment plants suffer from foam or mousse formation in which case specialist wastewater antifoams can be used – you should avoid using silicone based antifoams.
Dissolved air flotation (DAF Unit)
In dissolved air flotation or DAF units the treated effluent is purged with water supersaturated in air (known as the recycled water). The air is released with the drop in pressure and is caught in the flocs causing them to float. The sludge is then scraped off the top and the treated effluent is removed from the bottom.
Increasing the floc size increases flotation rates but can also cause poor fines capture and produce a float that is poorly compacted. If the floc is too large it may sink.
Typically operators should aim for a maximum floc size of between 1.5-2.25mm. It is sometimes possible to improve flotation by adding the flocculant at the same time as the recycled water, so that the bubbles get caught up in the forming floc rather than on the underneath of the floc.
Techniques for sludge dewatering
Common techniques used in the sludge dewatering process include:
Plate and frame filter press
Using a plate and frame filter press, the sludge is pumped into the press and filtered against a fine weave cloth. The pressure in the system may rise to 10-15 BAR at which point the pump will stop and the press is opened to drop the cake. Some inorganic sludge’s will dewater freely enough not to require a flocculant. However, using a flocculent can improve filtration rates, which may be a benefit.
Other sludge’s will normally require a flocculant and low molecular weight or structured polymers are the typically used.
This dewatering process usually produces dry cakes and can be a very labour intensive method.
- Activated sludge: 15-20% solids
- Digested/primary sludge’s: 25-35%
- Mineral sludge’s: 30-50%
Gravity belt thickener (GBT)
Gravity belt thickeners are used to thicken a sludge prior to digestion or to reduce the volume of waste tankered off site. GBT is basically the free drainage part of the belt press. Medium molecular weight and branched flocculants are typically used here.
When using a belt press the flocculated sludge is drained on to a coarse weave belt which is then passed between two belts and through a series of rollers to squeeze out the excess water. Low molecular weight or structured polymers are typically used.
Cake solids are usually a lot lower when compared against plate and frame filter press techniques, but tends to be a less labour intensive process.
When using a centrifuge the flocculated sludge is spun at high a G-force. The solids are spun to the outside of the bowl and conveyed by a scroll to the discharge end. The separated liquid (centrate) is then discharged at the other end of the centrifuge.
Cake solids can vary from those equivalent to a belt press to as high as a plate and frame press.
Due to the high shear forces created in this application it is often better to use high molecular weight or structured flocculants.
Structured wastewater polymers are available in emulsion form. These structured products have some of their charged sites hidden within the structure caused by cross-linking the chains together.
This structured type of product behaves very differently to linear polymers.
The solution tends to have a low viscosity so slight over dosing does not reduce the filtration rate as much as a linear product. However, when subjected to high shear the structure breaks up releasing more sites that improve the flocculation still further.
Structured polymers usually have to be applied at a higher dose than a linear product but can give significant improvement to throughput and cake solids.
All of these sludge dewatering techniques are commonly used in effluent treatment across a range of industrial, manufacturing and process sites and selecting the best or most appropriate to the individual application is important.
Selecting the best industrial effluent treatment product
Selecting the best chemical treatment products for an application is important and here we consider three of the most common scenarios.
Industrial wastewater treatment
It is not uncommon for some industrial plants to pre-treat their own process water prior to use. This is typically done by filtration using alum based water treatment products. However, partial replacement of the alum with a polyDADMAC can deliver benefits in increased filter runs and reduced sludge production and should be considered.
Typically high cationic flocculants are used in biological sludge applications. Structured flocculants may also perform very well on this type of sludge.
Mineral and metal containing effluents
Typically anionic flocculants should be considered for use where effluents contain minerals or metals. The metals are dropped out by adjusting the pH of the effluent to the pH required for optimum precipitation (lowest solubility) of the specific metal. If the residual metal content is still above the discharge consent set by the sewerage undertaker then the addition of a specialist metal treatment product, after pH correction, should be considered.
However, in industrial effluents where a coagulant has been used the resulting sludge may be reactive to both anionic and cationic polymers so both need to be tested.
It should also be noted that low pH effluents sometimes react to no-ionic flocculants.
Additionally, pH can be critical to successfully treating industrial effluents so operators should always test the pH, increasing or decreasing it between pH 4 to 11 if a suitable treatment cannot be found.
Specialist industrial wastewater and trade effluent solutions
Water Treatment Services offer a comprehensive range of industrial wastewater and trade effluent solutions. Find out how we can help reduce your costs, achieve regulatory compliance and improve environmental performance. Our experts can provide advice and full support to help you identify the most appropriate strategies for managing your wastewater and effluent streams.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers, wastewater specialists and technicians we offer cost effective environmental support solutions across the whole of the UK and Ireland.
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