Municipal Wastewater Treatment

This stage is primarily consisting of mechanical treatment and are usually the first processes of municipal wastewater treatment. At this stage the majority of larger solid wastewater constituents are removed from the water.

As a first step, screening systems are used to remove coarse impurities from the wastewater. By reducing the flow velocity in basins specially designed for this task - such as aerated grit chambers - coarser particles such as sand or gravel sink to the bottom. A considerable proportion of the undissolved organic load can also be removed from the water in this way. Along with the settlement of particulate matter, fats and oils can collect on the water surface and be removed separately.

Well-functioning mechanical treatment is of great importance for wastewater treatment plants. In addition to the general improvement in water quality, the removal of impurities is essential for the safe operation of downstream treatment stages.

In addition to screens, grit and grease traps, mechanical removal of solids also occurs during primary clarification. Solids settle out at this stage and are removed as primary sludge and then pumped for sludge treatment.

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Secondary treatment removes the soluble organic matter, nutrients such as nitrogen and phosphorus, and most of the suspended solids that escape primary treatment. Most often, biological processes are used in which microbes metabolise organic compounds and nutrients to grow and reproduce. The two most common biological secondary treatment processes are attached growth and suspended growth systems. A suspended growth process fosters the growth of suspended microorganism flocs from individual organisms already present in the wastewater and in the return activated sludge. The flocs contain organisms that can remove the pollutants through aerobic, anoxic, and anaerobic environments. Once the pollutants are removed, the flocs are sent to a secondary clarification process where they separate from the water via gravity. A portion of sludge in the bottom of the secondary clarifier is then directed back upstream to blend with the primary effluent (Return Activated Sludge) to create mixed liquor. The remainder of the sludge is removed from the process (Waste Activated Sludge) to create the ideal ecology of microorganisms. Attached growth systems rely on the microorganisms to attach to a media and create a biofilm. The settled sewage is either mixed or sprinkled over the biofilm coated media where the microorganisms remove the pollutants. Like the suspended growth process, biofilm fragments and suspended flocs are sent to a secondary clarifier for separation where sludge is recycled and wasted, and clean water is discharged to the next process.

For biological treatment to function efficiently, organisms require nutrients in a balanced ratio, including carbon, nitrogen, and phosphorus (referenced as C:N:P), as well as trace elements including iron, copper, zinc, nickel, manganese, potassium, sulphur, and other components which are typically present in wastewater. The commonly accepted C:N:P Ratio is 100:5:1, although some facilities thrive outside of this ratio, while others experience polysaccharide slime formation or filamentous bacteria growth that inhibit the biology and settling in the secondary clarifier.

Multiple biological processes can be employed to complete secondary treatment, including plug flow aeration basins, complete mix aeration tanks, sequencing batch reactors, oxidation ditches, trickling filters, moving bed biological reactors, integrated fixed film activated sludge, and others.

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Phosphorus is of crucial importance for the biological growth of microorganisms and is often a limiting nutrient in biological systems. Particularly in sensitive ecosystems, the input of phosphorus compounds via treated wastewater can therefore have a considerable influence on the eutrophication of water bodies.

For this reason, tight consents are often placed on the removal of phosphorus, which cannot usually be achieved with the biological stage alone.

Chemical phosphate elimination is therefore widely used to achieve the effective removal of phosphorus compounds. In this process, iron or aluminium salts are added to the wastewater, which are able to convert phosphate into insoluble compounds and thus remove them from the treatment stream.

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In tertiary treatment, techniques such as filtration, disinfection, carbon absorption, and other processes are used to remove the remaining organic load, suspended or dissolved solids, pathogens and heavy metals that pass through other treatment processes. Also called effluent polishing, tertiary treatment raises the effluent quality to the level suitable to its intended use, whether for discharge into lakes, rivers, or oceans, re-use as non-crop irrigation (parks, golf courses, greenways, etc), groundwater recharge, or in certain cases, as influent to drinking water plants.

Wastewater plant effluent must be monitored to ensure compliance with effluent permit limits, which vary based on jurisdiction and country.

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The use of additional processes to achieve advanced wastewater treatment is becoming increasingly important. These processes are often termed quaternary treatment and to further improve the quality of the effluent from municipal wastewater treatment plants.

The focus here is particularly on organic trace substances, such as pharmaceutical residues, household, and industrial chemicals, which are sometimes only partially removed in preceding conventional process stages.

In addition to the removal of chemical contaminants, a further focus of advanced wastewater treatment can also be to reduce pathogens and prevent the discharge of antibiotic-resistant organisms.

The choice of the process used in advanced wastewater treatment stages is dictated by treatment objectives. Adsorption processes using activated carbon (granulated or powdered) and oxidation processes with ozone are often used to remove organic trace substances.

The method for handling the sludge removed from the process depends on the volume of solids as well as other site-specific conditions. Aerobic digestion is often used by facilities with less than 30 million litres per day of inflow. Waste Activated Sludge and if present, Primary Sludge, are added to an aerated reactor where microorganisms feast on the organics and microorganisms present in the sludge to reduce the volatile solids content and the overall mass of sludge. Anaerobic digestion is typically used at facilities greater than 30 million litres per day of inflow and involves the use of sealed reactors to create an anaerobic environment for different organisms to feast on the organics and microorganisms in the sludge through the processes of acetogenesis and methanogenesis. The methane formed by anaerobic digestion can be used to fuel boilers to heat the digester, flared, or cleaned and repurposed as a green energy source.

Thickening involves concentrating the sludge by removing a percentage of the liquid portion by adding polymer compounds and is often employed before anaerobic digestion. Dewatering with belt presses, centrifuges or other means further concentrates sludge into a cake. The cake can be further dried, or simply disposed of through land application or landfills.

A growing field of expertise within the wastewater treatment industry, Water Intelligence Systems use digital technologies, advanced sensors, controllers, and algorithms to allow plant operators to increase efficiencies leading to overall cost savings in running the plant.

With these systems, data on flow rate, water composition, dissolved oxygen, nutrient levels, and other factors are more easily tracked and verified. This leads to more responsive treatment options, automation capabilities, and data visualisation and report generation.

Using data-driven insights into water quality, flow rate, and other factors, operators can reduce overtreatment (chemicals as well as aeration blower run time) while knowing their plant will remain within compliance limits.

As regulations become more stringent and plants need to find cost-savings, Water Intelligence Systems such as Claros will become more important and widely adopted. And with Hach’s growing suite of Claros-Enabled sensors, controllers, devices, Process Management systems, and lab equipment, operators can configure a system that fits the unique requirements of any plant.