In the processing sectors, industrial wastewater is a big problem. In fact, due to the high costs or challenges connected with industrial wastewater solutions and treatments, some processing projects have not been realised. Many countries have undertaken large-scale environmental programmes, resulting in stringent environmental rules on industrial wastewater disposal. While operators may have installed industrial wastewater treatment systems to fulfil local laws when they were created, they eventually required costly upgrades to meet new tougher limits and tighter restrictions. Even after extensive adjustments, some could not achieve those tight limits.
Industrial wastewater typically has varied quantities of organic and inorganic materials. Many materials are poisonous, mutagenic, carcinogenic, or nearly indegradable. This means that the wastewater contains a variety of chemicals that are difficult to breakdown.
Because primary treatment involves the removal of solids, particles, and oils from an industrial wastewater stream, it typically includes fundamental physical procedures and solid/oil separations such as primary clarifiers, oil separators, and screens. Secondary treatment is typically the heart of the treatment unit, breaking down suspended and residual organics and chemicals. Secondary treatment often entails the biological (bacterial) breakdown of contaminants and pollutants. Aerated activated sludge treatment has long been recognised as one of the most effective secondary treatment methods. It is easy, inexpensive, and effective.
Many contaminants, including soluble biodegradable organic pollutants, have been found to be effectively removed by combining anaerobic and aerobic treatment techniques. Membrane-based methods are increasingly being used in industrial wastewater treatment. Chemical oxidation approaches for wastewater treatment are also becoming more popular as treated wastewater restrictions tighten.
In current industrial wastewater treatment plants, both traditional chemical treatment and sophisticated oxidation methods are utilised. Tertiary treatment often involves final filtering, polishing, and finishing stages, such as commonly used activated carbon filters.
This article discusses industrial wastewater treatment technologies used in the processing industries, such as physicochemical, biological, and advanced oxidation processes.
Gravity separation and skimming, dissolved air flotation (DAF), de-emulsification, coagulation, and flocculation have all been used in the past to treat oily wastewater. Gravity separation, followed by skimming, is an efficient method for recovering free oil from industrial wastewater.
API separators and variants have gained universal support as a useful, low-cost primary therapy step. The API oil-water separator separates oil and suspended particles from wastewater. Smaller oil droplets and emulsions cannot be removed by an API separator or other simple oil-water separator. Sedimentation in a primary clarifier can effectively remove oil that adheres to the surface of solid particles.
DAF is a highly successful technology for treating tiny oil droplets and emulsions. Air is used by DAF to increase the buoyancy of tiny oil droplets and improve separation. De-emulsification by chemicals, heat energy, or both is used to remove emulsified oil from DAF. To facilitate separation, DAF devices often use chemicals to enhance coagulation and increase flock size. In most cases, emulsified oil in industrial effluent is chemically processed to destabilise the emulsion before gravity separation.
Heat is frequently used to reduce viscosity, emphasise density differences, and degrade the interfacial coatings that stabilise the oil phase. After acidification and the addition of cationic polymer/alum to neutralise the negative charge on oil droplets, the pH is raised to the alkaline zone to stimulate flock formation of the inorganic salt. The resulting flock containing the adsorbed oil is separated, followed by sludge thickening and dewatering.
Flocculation and coagulation:
Sedimentation is a procedure that is used in the majority of industrial wastewater treatment plants. Sedimentation, also known as clarifying, is a wastewater treatment procedure in which the wastewater velocity is reduced below the suspension velocity and the suspended particles settle out of the wastewater due to gravity. Sludge is made up of settled solids, while scum is made up of floating solids. Industrial wastewater flows from the sedimentation tank to the next stage of treatment via an effluent weir. Retention time, temperature, tank features, and other elements all influence the process’s efficiency or performance.
However, without coagulation/flocculation, sedimentation can only remove coarse suspended matter that settles quickly out of the wastewater without the use of chemicals. This form of sedimentation often occurs at the start of the treatment process in a reservoir, sedimentation, or clarity tank. Coagulation/flocculation is based on the addition of chemical products (coagulants) that increase sedimentation in clarity tanks. Coagulants are inorganic or organic substances such as aluminium sulphate, aluminium hydroxide chloride, or cationic polymers with a high molecular weight. At this step in the treatment process, the use of coagulant is intended to remove about 90% of the suspended particles from industrial wastewater.
Biotreatment of wastewater for processing facilities:
Biological processes are primarily concerned with organic contaminants. Over the last century, microbial-based approaches have been utilised to clean industrial wastewater. The advancement of these technologies has resulted in the successful annihilation of waste elements that are readily biodegradable under aerobic conditions.
Aerobic degradation in the presence of oxygen can be a relatively easy, low-cost, and ecologically acceptable method of waste breakdown. Temperature, moisture, pH, nutrients, and aeration rate that the bacterial culture is exposed to are critical factors in the optimal degradation of the selected substrate, with temperature and aeration being two of the most critical parameters that determine the degradation rates by the microorganism.
Any viable microbial action — aerobic, anaerobic, or anoxic — can eliminate soluble organic sources of biochemical oxygen demand (BOD). Aerobic processes, on the other hand, are commonly used as the primary mechanism of BOD reduction in wastewater because aerobic microbial responses are quick — typically 10 times faster than anaerobic microbial reactions. As a result, aerobic reactors can be built very small and open to the atmosphere, providing the most cost-effective method of BOD reduction.
The main disadvantage of aerobic bioprocesses for wastewater treatment over anaerobic processes is the enormous amount of sludge produced. Because the biomass yield (mass of cell produced per unit mass of biodegradable organic matter) for aerobic bacteria is extremely high — nearly 3 to 4 times more than the yield for anaerobic organisms — a comparatively high accumulation of biomass occurs in the aerobic bioreactor. The sludge contained in the reactor effluent may contain residual BOD that must be reduced in another step before being disposed of as solid waste.
During the aerobic degradation process, microorganisms can use a variety of mechanisms, including the attack on xenobiotics by organic acids produced by the microorganisms, the production of noxious compounds such as hydrogen sulphide, and the production of chelating agents, which can increase the solubility of any insoluble xenobiotics, making them more available to the microorganisms, as well as mechanical degradation.
Some industrial effluent can be hazardous to the microorganisms found in a traditional activated sludge reactor. Contaminations and chemicals found in these wastewater streams are hazardous and cannot be used as a sole carbon source by microorganisms. As a result, inhibition of microbe development by these components is critical in the degradation process since it can cause the treatment system to fail.
The key to successful industrial wastewater biotreatment technology is to change or optimise the cell and substrate contact time so that biodegradation may occur in a fair amount of time and the potential toxicity of the wastewater to bacteria and microflora is reduced.
“Anaerobic reactors vary from aerobic reactors chiefly in that the former must be closed to prevent oxygen from entering the system and interfering with anaerobic metabolism.”
Activated sludge treatment technologies are widely employed in the treatment of industrial wastewater. Membrane bioreactors (MBR) inoculated with activated sludge have been found to successfully treat high-strength organic effluent. The two-phase partitioning reactor, on the other hand, has proven to be effective with harmful substrates.
Anaerobic reactors vary from aerobic reactors principally in that the former must be closed to prevent oxygen from entering the system and interfering with anaerobic metabolism. An anaerobic reactor should have an appropriate vent or collection mechanism to remove the gases produced during anaerobiosis (mostly methane and carbon dioxide).
There are various advantages to anaerobic microbial processes:
- Sludge production rate is reduced.
- Operable at greater BOD and harmful levels in the influent.
- There are no expenses connected with providing oxygen to the reactor.
- Methane production as a beneficial byproduct (biogas).
Anaerobic processes, on the other hand, have greater capital and operational costs than aerobic processes because anaerobic systems must be closed and typically heated. Anaerobic bioprocesses for hazardous wastewater treatment are often limited to low flow rate streams, unless further accommodations are made.
Anaerobic digestion is made up of multiple interdependent, complex sequential and parallel biological reactions in which the products of one set of bacteria serve as substrates for the next, resulting in the transformation of organic matter mostly into methane and carbon dioxide. Hydrolysis/liquefaction, acidogenesis, acetogenesis, and methanogenesis are the four stages of anaerobic digestion.
To guarantee a balanced digestive process, the various biological conversion activities must be properly connected throughout the process to prevent the accumulation of any intermediates in the system. For industrial wastewater treatment, anaerobic reactors such as up-flow anaerobic sludge blanket (UASB) and anaerobic sequencing batch reactor (ASBR) have been employed.
Advanced wastewater oxidation processes for processing facilities:
By definition, oxidation is the transfer of electrons from one substance to another, resulting in a potential defined in volts known as the normalised hydrogen electrode. The oxidation potentials of the various chemicals are derived from this. Chemical oxidation looks to be a law-compliant alternative for treated wastewater.
It is typically used following a secondary treatment to destroy nonbiodegradable compounds. The chemical oxygen demand is a reference parameter when employing chemical oxidation as a treatment technique (COD). Typically, these methods can treat wastewater with relatively low COD values since greater COD contents would need the use of excessive volumes of expensive reactants. Chemical oxidation processes are classified into two types:
Traditional chemical therapy:
Processes of advanced oxidation (AOPs)
AOPs are wastewater treatment methods that entail the formation of highly reactive radicals (particularly hydroxyl radicals) in sufficient quantities for wastewater purification at near ambient temperature and pressure. These treatment approaches are promising for the remediation of nonbiodegradable organic contaminants in contaminated ground, surface, and waste water. Hydroxyl radicals are extremely reactive entities that damage the majority of organic compounds.
AOPs have some technological and economic restrictions in their application. There are significant limits in applying them to whole-site wastewater flow or using them in continuous operation. AOPs are essential for some large critical treatment facilities to deal with peak COD in order to meet rigorous treatment restrictions. These units are typically utilised following biotreatment. AOPs are also very effective at converting recalcitrant compounds into intermediates amenable to biological oxidation via recirculation to the inlet of the biological unit or, even better, completely mineralizing these compounds when applied as a final polishing step in the outlet of a biological treatment facility.
Among AOPs, Fenton’s reagent has been shown to be efficient in treating industrial wastewater components such as aromatic amines, dyes, and other chemicals such as pesticides and surfactants. One advantage of Fenton’s reagent is that it does not require any energy to activate hydrogen peroxide.