Treatment of Wastewater
The treatment of wastewater which can comprise of household sewage and effluent from industrial plants is a process by which pollutants are removed. There are many processes used to remove these pollutants, including chemical and biological processes as well as physical ones. The resulting treated effluent should be clean enough to be discharged back into the environment or reused and the leftover solid waste disposed of. Effluent and sewage can be treated in various ways, one of them is a municipal treatment plant. At this plant, the wastewater undergoes various stages of treatment including pre-treatment, primary, secondary and tertiary.
During pre-treatment, materials that are easily removed are separated or screened out, including grit (sand and gravel), oils, fats and greases, and larger objects.
This is generally the removal of suspended solids by settling and floating. In the process of primary treatment, the effluent is transported through primary clarifiers, which are large tanks where sludge sinks to the bottom and greases and oils rise to the top (where they can be skimmed off). During this process, the solids and liquids separate, with the solids settling to the bottom of the tanks. There they can be gathered and moved and treated separately. Chemicals are sometimes added to help remove small suspended solid particles.
During the secondary treatment, aerobic biological processes are used to break down the biological content of the effluent (human or food waste and soaps, among other things). Also in this phase, there is rapid mixing and agitation which can give rise to plenty of foam generated. The biological process chosen depends on the quantity of wastewater and the biodegradability of water.
During the tertiary treatment, the quality of the effluent is improved through various disinfection processes such as chlorination, use of ozone or ultraviolet light, as well as some other methods such as sand filtration or putting effluent in highly aerobic lagoons or constructed wetlands. Chlorine is abundant and cost-effective but can potentially create carcinogenic compounds and/or effluent that is toxic to aquatic species (unless dechlorinated). Ultraviolet light creates water that's healthier for the environment. It's deadly to bacteria and viruses because it alters their genetic structure, making them incapable of reproducing. It isn’t always effective if the water is cloudy and the pathogens are protected from the ultraviolet light. Ozone destroys pathogens because of its instability and reactive nature cause it to oxidize most organic materials. Sometimes, a combination of tertiary treatments is required.
The causes of foam in a wastewater treatment plant are varied and can be caused by chemical, biological or mechanical processes. In secondary treatment systems, foam is normally caused by biological activity. It can also result from mechanical action, chemical contamination, surfactants in the influent or some polymer treatments. Apart from the cause, foaming in secondary treatment and final effluent areas can lead to environmental, health and safety issues.
Excessive foam in the aeration basin can give a number of problems. A blanket of foam here can interfere with the oxygen uptake of the microorganisms and hinder BOD reduction.
There is a wide range of defoamers — esterified polyols, aqueous emulsions of fatty alcohols and silicone-based products — to treat foaming in the oxygenation basin.
These defoamers have been specially formulated to not interfere with dissolved oxygen content. These defoamers have a minor environmental impact due to low toxicity and good biodegradability. For most applications, light foam is not problematic, but excessive foam and/or dense foam can often lead to different processing problems. These problems range from false readings from control and measuring equipment (such as thermometers and level and mass flow transmitters). Also, poor mixing of reactants in chemical process tanks and products that get stuck in the foam can cause losses in production. Defoamers are used to limit, reduce, avoid or eliminate foaming.
The mechanism of many foam control products is to add hydrocarbon or silicone oils that are hydrophobic. These products work with surfactant induced foams and polysaccharide induced foams caused by nutrient deficiency. However, foaming due to Nocardia-form bacteria and Microthrix filament type bacteria are quite different in nature and require a defoamer that is chemically different. These filaments are hydrophobic and are typically present when high FOG ( fat, oil and grease) concentrations and/or fatty acids are present at significant concentrations. Hydrocarbon and silicone oils can actually make the problem worse.
Glycol based defoamers without mineral oil or silicone oils can be optimized for this type of microbial foam. The defoamer helps to break-up the tangled filament mass in the foam and disperse the filaments to accelerate washout.
Bulking sludge and foam are two undesirable, complicated and unpredictable challenges for many wastewater treatment plants (WWTP). The filaments causing bulking sludge tend to float and thereby produce large amounts of foam which can vary in depth and can extend throughout the biological circuit, as well as to anaerobic digesters and dewatering units.
The traditional approach is to continuously dose anti-foam into the process at a rate typically set to cope with foam during peak demand. However, this quantity is, mostly, far too high as in some systems there is little or no foam generation for most of the time. Alternatively, the addition of chemicals is done randomly, in far too large quantities and too late. In some cases, a WWTP can suffer for years without finding a sustainable solution to the problem. The chemicals used also add to the foaming problem and the cost of chemicals and energy through unnecessary pumping can become substantial.
The types of defoamers that are utilised in wastewater plants are varied, from silicone based, mineral-oil based and polyalkylene glycol type. As mentioned earlier defoamers containing hydrophobic properties can worsen the foaming issue in bacteria based ones. Lankem is actively developing a series of bio-based ones for these types of foam.
The Dfoam AX1 is a mineral-oil based defoamer and can be used as a general purpose one at dosing levels at 5 – 50ppm.
Flow diagram of a typical wastewater treatment plant