Contemporary Methods for Treating Sewage Wastewater: From Traditional Approaches to Advance Oxidation Processes

Wastewater treatment is a necessary part of our daily lives because it helps us meet the needs of human life. In the last few years, sewage treatment has gotten a lot better in developing countries. This has made people worry about textiles polluting wastewater.

Sewage wastewater usually has a lot of different pollutants in it, such as heavy metals, bacteria, and solids that are floating around. Sewage wastewater is one of the most polluted types of water that needs to be cleaned up because it has a lot of colour, a high pH, a lot of organics that don't break down easily, a lot of stability, and a lot of turbidity. If not treated, this sewage wastewater could be very harmful to people and the environment. 

Domestic wastewater: Toilets, showers, sinks, and laundry all produce wastewater in homes and apartments. Despite being more than 99.9% water, this water contains disease-causing bacteria, plant nutrients, and putrescible organic compounds.

• Commercial wastewater: Waste from restaurants, lodging facilities, and office buildings frequently contains oils, grease, and commercial chemicals besides food residue.

• Industrial wastewater: Waste with particular compounds and greater concentrations of pollutants is produced by mining operations, manufacturing processes, chemical plants, food processing, and other industrial activities.

A typical wastewater plant involves three main stages: primary, secondary, and tertiary treatment. 

The sewage treatment process commences with preliminary treatment, where raw influent is passed through a mechanical bar screen with 10-20 mm spacing to remove coarse solids, rags, and debris, followed by a grit chamber operation at a flow velocity of 0.3-0.6 m/s to settle organic (depending on plant operation and area) and inorganic particles such as sand and grit, thereby preventing abrasion in downstream equipment. The primary clarifiers, typically with a hydraulic retention time (HRT) of 2-4 hrs, allow suspended solids to settle and reduce the biochemical oxygen sludge processing. Microbial oxidation of dissolved and colloidal organic matter is facilitated in an aeration tank by maintaining dissolved oxygen (DO) levels of 2-4 ppm and an MLSS (mixed liquor suspended solids) concentration of 2000-3500 ppm. After that, the mixed liquor is sent to secondary clarifiers for a sludge retention time (SRT) of eight to fifteen days, which guarantees that there is enough biomass for stable biodegradation. After tertiary treatment, such as sand filtration or activated carbon adsorption, the secondary clarifier's effluent can be disinfected with UV irradiation or chlorine (1-3 ppm, 30 min contact time) to achieve pathogen inactivation and discharge standard compliance. Sludge from primary and secondary clarifiers is simultaneously treated in sludge thickeners and anaerobic digesters under mesophilic conditions (35–38°C, 15–20 days retention). This is followed by mechanical dewatering to bring the moisture content down to 20–30%, allowing for resource recovery or safe disposal. Significant BOD, COD, SS, and pathogen load reductions are guaranteed by this integrated process, producing environmentally acceptable effluent that can be reused or discharged. 

It is necessary to remove large and coarse materials from wastewater through a screening process because they have the potential to harm machinery or clog pipes. Large waste materials like paper, plastic, and cloth are kept out of the way by metal screens. Following the grid removal process, small stones, gravel, and sand were allowed to settle into the chamber. Flow equalisation is an optional step that balances the treatment system to guarantee peak performance.

Sitting is a step in the primary treatment process that constantly eliminates organic debris and floating sediments from wastewater, allowing lighter materials, such as oil and grease, to float to the top and be skimmed off while heavier contents, such as sludge, fall to the bottom.

Bar Filter: A bar filter is a kind of screen or mechanical screening device used in the preliminary treatment stage of a sewage treatment plant. Its main purpose is to remove large solids and debris from raw sewage to protect the downstream equipment from clogging, abrasion, or damage. 

Grit Chamber: It is a hydraulic structure used in the preliminary treatment stage in a plant to remove heavy inorganic particles such as sand, gravel, and grit from raw sewage. The purpose is to prevent abrasion, sedimentation, and blockages in downstream equipment like pumps, aerators, and clarifiers. 

Types of Grit Chambers: 

1. Horizontal Flow Grit Chamber 

2. Aerated Grit Chamber 

3. Vortex Grit Chamber  

To remove dissolved and biodegradable organic matter using microorganisms. 

The activated sludge process involves pumping oxygen into aeration tanks, where organic matter is broken down by bacteria. 

Trickling Filter: Wastewater is sprayed over rocks or plastic media coated with microbes. 

Oxidation ditch or sequencing batch reactor: modern variations that improve control and efficiency. So, in this way, approximately 85-95% of organic pollutants are removed. 

Now, coming to tertiary treatment for wastewater treatment, where to polish the effluent and remove remaining contaminants. 

To filter, techniques, sand, or membrane filters remove fine particles. 

Disinfection: usually with chlorine, ozone, or UV light to kill harmful microorganisms. 

Nutrient removal: Some plants remove nitrogen and phosphorus to prevent water pollution. 

 

Thermodynamics: Saturation Pressure

Saturation pressure and temperature in a physicochemical process are essential active factors in the thermodynamic system to identify deviations and aid in the calculation of liquid holdups, levels, and flow rate in the chemical reactor. The first indicator that is essential to process plant operation is pressure, which gives information about the range of phase flow from one place to another. Each pipe flow has a pressure gauge or transducer placed to properly calibrate and navigate pressure ranges. Manometers, piezoresistive, piezoelectric, piezocapacitive, and Bourdon pressure gauges are among the several types of pressure gauges. Installing precise pressure gauges with a particular unit is necessary to determine the saturation pressure measurement. 

This graph is a phase diagram that illustrates the relationship between a substance's temperature (in Kelvin) and pressure (on a logarithmic scale, in MPa), as well as its distinct states of matter (solid, liquid, and vapour). The black curves represent phase boundaries, where two phases exist in equilibrium. The liquid–vapour equilibrium curve, which terminates at the critical point (represented as a filled circle at high temperature and pressure), is represented by the curved line extending to the right. The solid and vapour areas are separated by the sublimation curve on the left, while the solid and liquid areas are separated by the melting curve on the right. The triple point, a critical location where the solid, liquid, and vapour phases are in equilibrium, is denoted by the grey circle. The relative quantities of each phase in an entirely liquid system are illustrated in the inset bar graph. This graphic elucidates the manner in which the state of a substance is influenced by temperature and pressure.

The temperature-entropy (T-s) diagram for carbon dioxide is a thermodynamic property chart that is often used in the study of refrigeration systems and heat pump cycles. The temperature (in °C) is shown on the vertical axis (ordinate) of this diagram, while the entropy (measured in kJ·kg⁻¹·K⁻¹) is shown on the horizontal axis (abscissa). The saturated zone is defined as a specific region/point that is bordered on the right by the saturated vapour line and on the left by the saturated liquid line. This region includes the two-phase mixing region, where the refrigerant exists in a vapour-liquid equilibrium. Within this region, the vapour percentage is indicated by the constant-quality lines (dashed curves). The performance assessment and energy balance of the cycle rely on both isobars (constant-vaporisation lines) and isenthalpies (constant-enthalpy lines), which are illustrated by the blue and green curves, respectively. 

At (~31 °C), where critically lower, which denotes the apex of the saturation region, is a crucial observation for CO₂. After this, the refrigerant converts into the supercritical region, where heat rejection occurs at supercritical pressures instead of through typical condensation, and phase change does not take place. Diagram differs from traditional refrigerants due to its thermophysical behaviour, which requires the usage of trans-critical cycles in CO₂-based refrigeration and heat pump systems. So that, engineers may accurately evaluate and optimise CO₂ thermodynamic cycles by using the T–s diagram to illustrate compression (an almost isentropic process), heat rejection, throttling (isenthalpic expansion), and heat absorption. The Temperature–Entropy (T–s) graph demonstrates that thermodynamic processes of a CO₂-based vapour compression refrigeration cycle are going quite well. The cycle starts with isentropic compression of low-pressure vapour (state 1 → 2). During this process, the temperature and pressure rise quickly and move up the T–s plane, while the refrigerant entropy stays virtually the same. Because of its lower critical existence temperature (~31°C), the supercritical region is where the high-pressure vapour often occurs after entering the gas cooler/heat rejection process (states 2–3). As heat is rejected to the environment at nearly constant pressure during this operation, entropy falls, moving the state to the left on the diagram. 

The refrigerant is then processed through the isenthalpic throttling/expansion (state 3 → 4), which is observed on the T–s chart as a vertical line pointing downward since enthalpy remains constant while entropy rises. In subcritical operation, this mechanism moves the refrigerant into the two-phase region; in trans-critical operation, it moves into a lower enthalpy superheater/supercritical state. The refrigerant moves rightward across the two-phase region (quality increases from liquid-rich to vapour-rich mixture) during the last stage, heat absorption in the evaporator (state 4 → 1), where it evaporates at constant pressure until it reaches saturated or slightly super-heated vapour conditions at the compressor inlet. This diagram highlights the unique thermodynamic properties of CO₂, particularly its inability to condense under typical ambient conditions. This characteristic necessitates the use of a trans-critical cycle arrangement. For chemical and refrigeration engineers, understanding the T-s diagram is essential for improving the design and operational efficiency of these applications. 

Note: - The diagram serves as a vital tool for analysing work input, heat transfer, and overall cycle efficiency.