6 wastewater treatment systems

Article details

6 wastewater treatment systems

This article describes 6 wastewater treatment system types:                           

1. Stabilisation pond treatment ( Stabilization Pond)

2. Aerated lagoon treatment ( Aerated Lagoon or AL)

3. Constructed wetland treatment ( Constructed Wetland)

4. Activated sludge treatment ( Activated Sludge Process)

5. Oxidation ditch treatment ( Oxidation Ditch)

6. Rotating biological contactor treatment ( Rotating Biological Contactor ; RBC)

Why wastewater treatment matters

A wastewater treatment plant collects wastewater from homes, commercial premises, industries and institutions for treatment processes that remove pollutants, improve water quality and prevent harm to rivers, canals, natural water sources and the surrounding environment. Treated effluent is discharged to public waters, while some can be reused in agriculture, industry and other applications.

Although water is repeatedly reused through its natural cycle and has a self-cleaning process ( Self Purification) that process has limited capacity in each water body. Wastewater treatment therefore reduces the burden on natural self-purification and helps prevent pollutants contaminating raw water used for drinking-water production.

Wastewater collection

Sewerage is a complex network of pipes that collects wastewater from residences, industry, commercial businesses and institutions. Underground sewers carry it to treatment before discharge to the environment. Wastewater volumes are broadly related to community water use, and inflows vary with daily usage patterns and seasons. The sewer network must accommodate the full inflow without leakage or community flooding.

Wastewater treatment

Selecting a wastewater treatment system depends on wastewater characteristics, required treatment level, local conditions, construction investment, operating and maintenance costs, and the available land area. The selected system should suit each locality's environment. Treatment can be classified by the mechanism used to remove impurities, as follows.

1. Physical treatment ( Physical Treatment) : This separates impurities from wastewater, such as large solids, paper, plastic, food scraps, gravel, sand, grease and oil. Physical treatment equipment includes screens, grit chambers, grease and oil traps, and settling tanks. Its main purpose is to reduce total solids in the wastewater.

2. Chemical treatment ( Chemical Treatment) : Chemical treatment uses reactions with wastewater impurities. It is used where wastewater has an excessively high or low pH, toxic substances, heavy metals, suspended solids that are difficult to settle, dissolved fats and oils, excessive nitrogen or phosphorus, or pathogens. Equipment includes rapid-mix tanks, slow-mix tanks, settling tanks, filters and disinfection tanks.

3. Biological treatment ( Biological Treatment) : This uses biological processes or microorganisms to remove impurities, especially organic carbon, nitrogen and phosphorus. These contaminants serve as food and energy for microbial growth in a culture tank, reducing the pollutant load. The microorganisms may be aerobic ( Aerobic Organisms) or anaerobic ( Anaerobic Organisms) Biological wastewater treatment systems include activated sludge ( Activate Sludge, AS) rotating biological contactors ( Rotating Biological Contactor, RBC) oxidation ditches ( Oxidation Ditch, OD) aerated lagoons ( Aerated Lagoon, AL) trickling filters ( Trickling Filter) wastewater treatment ponds ( Stabilization Pond) UASB systems ( Upflow Anaerobic Sludge Blanket, UASB) and anaerobic filters ( Anaerobic Filter, AF) among others.

Wastewater treatment can also be divided into the following stages:

1. Preliminary treatment ( Preliminary Treatment) and primary treatment ( Primary Treatment) : separate sand, gravel and large solids from wastewater. Equipment includes coarse screens ( Coarse Screen) fine screens ( Fine Screen) grit chambers ( Grit Chamber) primary settling tanks ( Primary Sedimentation Tank) and scum removal equipment ( Skimming Devices) The source describes this stage as removing 50 - 70 percent of suspended solids and 25 - 40 percent of organic matter measured as BOD.

2. Secondary treatment ( Secondary Treatment) : Wastewater after preliminary and primary treatment still contains small suspended solids and dissolved and undissolved organic matter. Secondary treatment, also called biological treatment ( Biological Treatment) cultivates microorganisms under controlled conditions so they consume organic matter faster than in nature. Microbial sludge is separated from the effluent in a settling tank ( Secondary Sedimentation Tank) improving effluent quality. The water then passes through disinfection ( Disinfection) to control disease-causing microorganisms before discharge to natural water bodies or reuse ( Reuse) The source states that this treatment stage can remove more than 80 percent of suspended solids and organic matter measured as BOD.

3. Advanced treatment ( Advance Treatment or Tertiary Treatment) : This removes nutrients such as nitrogen and phosphorus, colour, difficult-to-settle suspended matter and other substances not removed by secondary treatment. The aim is to further improve water quality sufficiently for reuse ( Recycle) It also helps prevent excessive algal growth that can cause water deterioration, addresses objectionable colour, and tackles other problems that secondary treatment cannot remove. Advanced treatment processes include:

Phosphorus removal using either chemical or biological processes.

Nitrogen removal using chemical or biological processes. Biological nitrogen removal has 2 stages: conversion of ammonia nitrogen to nitrate under aerobic conditions, called nitrification ( Nitrification)" and conversion of nitrate to nitrogen gas under anoxic conditions, called denitrification ( Denitrification)"

Combined biological phosphorus and nitrogen removal uses aerobic and non-aerobic processes for nitrogen removal through nitrification and denitrification, together with luxury phosphorus uptake ( Phosphours Luxuty Uptake) which also requires an anaerobic process followed by an aerobic process. Application requires a thorough understanding of these processes.

Filtration ( Filtration) physically removes unwanted substances, such as suspended matter that is difficult to settle.

 

Adsorption ( Adsorption) removes organic substances from wastewater by attachment to solid surfaces. The same principle can remove odours or gases.

Sludge treatment ( Sludge Treatment)

Biological wastewater treatment always produces microbial sludge as microorganisms grow while consuming organic matter. The sludge must be treated to prevent putrefaction and odour, avoid additional pollution and destroy pathogens. Removing water to reduce sludge volume also makes transport for disposal or beneficial use easier. Main sludge treatment processes include:

1. Thickening ( Thickener) using thickening tanks based on settling ( Sedimentation) or flotation ( Floatation) to reduce sludge volume before further treatment.

2. Sludge stabilisation ( Stabilization) using aerobic or anaerobic digestion to reduce organic matter and stabilise the sludge so that it can be disposed of without putrefaction and odour.

3. Sludge conditioning ( Conditioning) to prepare it for subsequent beneficial uses such as fertiliser or agricultural soil improvement.

4. Dewatering ( Dewatering) reduces sludge volume before landfill, incineration or other uses and makes transport easier. Dewatering equipment includes vacuum filters ( Vacuum filter) filter presses ( Filter press) centrifuges ( Centrifuge) and sludge drying beds ( Sludge drying bed)

Sludge disposal ( Sludge Disposal)

After sludge has been stabilised, its odour controlled and its volume reduced for easier transport, the next step is disposal by an appropriate method. Disposal methods described by the source include:

Landfill ( Landfill): placing sludge at a prepared site and covering it with another layer of soil.

Composting ( Composting) : further composting sludge for use as fertiliser. This reuses the sludge's plant nutrients, including nitrogen, phosphorus and various minerals.

Incineration ( Incineration) : burning nearly dry sludge with at least 40 percent solids where it cannot be used as fertiliser or disposed of by landfill.