Wastewater treatment, collection and discharge are essential to protect human health, the environment and surrounding water quality. Before it can be treated, wastewater needs to be collected from sewer networks servicing homes, municipal, commercial and industrial premises, including rainwater run-off from roads and other impermeable surfaces. Wastewater treatment is also evolving. Historically it was designed to clean up wastewater before a cleaned up effluent could be discharged safely into the surround area. Today, wastewater is being seen a valuable resource to generate: energy, nutrients and water for irrigation, industrial and even drinking purposes. This article provides everything you need to know about the different treatment stages and technologies involved in wastewater treatment.
Wastewater: creating sustainable value
Why is wastewater treatment important?
Today, around 80% of all wastewater is discharged into the world's waterways where it creates health, environmental and climate-related hazards, according to the IWA. Estimates suggest wastewater treatment capacity is currently 70% of the generated wastewater in high-income countries, and only 8% in low-income countries.
Furthermore, urbanisation further exacerbates this challenge with increasing wastewater generation, while at the same time using more of Earth's dwindling resources, according to the IWA.
The discharge of untreated effluent in water bodies does not only lead to eutrophication and human health risks, it also contributes significantly to Greenhouse Gas (GHG) emissions in the form of nitrous oxide and methane. Emissions from untreated sewage represents three times the emissions of conventional wastewater treatment.
The emissions from untreated sewage can represent a significant percentage of cities' global emissions, even when treatment coverage is still poor as in many emerging cities.
Wastewater management and adequate sewer systems play important roles in sanitation and disease prevention. It is vital to develop a system to manage community wastewater and sewage. Otherwise, wastewater can contaminate the local environment and drinking water supply, thereby increasing the risk of disease transmission.
Global access to safe water, adequate sanitation, and proper hygiene education can reduce illness and death from disease, leading to improved health, poverty reduction, and socio-economic development.
However, in many countries, proper wastewater management is not practiced due to lack of resources, investment, infrastructure, available technology, and space. Many countries are challenged to provide these basic necessities to their populations, leaving people at risk for water, sanitation, and hygiene (WASH)-related diseases.
In the 2030 Agenda for Sustainable Development, Goal 6 aims to guarantee sustainable management of, and access to, water and sanitation for all by 2030.
However, in 2015, three in ten people (2.1 billion) did not have access to safe drinking water and 4.5 billion people, or six in ten, had no safely managed sanitation facilities.
As well as safeguarding human health and environmental protection, modern wastewater treatment is helping to identify ways to create value from the materials, energy and water that is embedded in wastewater streams.
What is wastewater treatment?
Wastewater treatment involves the collection of wastewater often by thousands of kilometres of sewer pipes.
The size of collection systems varies depending on the region and country they serve. For example, the largest collection systems in the UK are linked to around 9000 wastewater treatment plants.
In some countries and areas, rainwater from roofs, roads and pavements is collected in a separate system, called a surface water sewer, which goes straight into river. Alternatively, wastewater and surface water are mixed together in combined sewers before wastewater treatment.
A recent development in the UK using fibre sensing cables is evaluating the measurement of flow, depth, temperature and structural integrity every five metres along a sewer pipe.
Once wastewater reaches treatment plants, four stages are involved, including:
- Preliminary treatment (pre treatment) – to remove grit and gravel and screening of large solids.
- Primary treatment – to settle larger suspended, generally organic, matter.
- Secondary treatment – to biologically break down and reduce residual organic matter.
- Tertiary treatment – to address different pollutants using different treatment processes.
During wastewater treatment, a mixture of solids and water is generated, known as sludge. According to the IWA, the volume of sludge produced in a WWTP is only about 1% (dewatered sludge is 0.5%) of the volume of influent wastewater to be treated. To manage WWTPs effectively and efficiently, it is absolutely necessary to extract waste sludge, including inert solids and excess biomass, in order to prevent their accumulation within the system.
In a WWTP the types of sludge produced are:
- primary sludge – produced by settleable solids removed from raw wastewater in primary settling; characterised by high putrescibility and good dewaterability when compared to biological sludge; Total solids content in primary sludge is in the range 2-5%.
- secondary sludge (also called biological sludge) – produced by biological processes such as activated sludge or biofilm systems; contains microorganisms grown on biodegradable matter (either soluble or particulate), endogenous residue and inert solids not removed in the primary settling (where a primary settler is present) or entering with the raw wastewater (where no primary settler is present); TS content in secondary sludge is in the range 0.5-1.5%.
- chemical sludge – produced by precipitation of specific substances (i.e. phosphorus) or suspended solids.
Wastewater pre treatment (preliminary treatment)
Pre-treatment is necessary to remove anything that might interfere with subsequent treatment. It can protect raw water lifting systems and pipelines against blockages, as well as other treatment equipment against abrasion and to generally remove anything that might interfere with subsequent treatment. They can also help to reduce abrasion of mechanical parts and extend the life of the sanitation infrastructure.
The following constitute pre treatment operations, although wastewater treatment plants can comprise one of more of the options, depending on raw water quality: bar screening; straining; comminution; grit removal; grease removal, frequently combined with grit removal; oil removal; by-product treatment: grit and grease; combined treatment of mains cleaning waste and of plant grit.
Advantages of pre-treatment include relatively low capital costs and low to moderate operating costs. This is coupled with the reduced risk of impairing subsequent conveyance and/or treatment technologies. Meanwhile, the major disadvantage is frequent maintenance required.
Often the first unit operation used at wastewater treatment plants, screening is used to remove objects to prevent damage and clogging of downstream equipment and piping. Both coarse screens and fine screens can be used in some modern wastewater treatment plants.
Coarse screens remove large solids, rags, and debris from wastewater, and typically have openings of 6 mm (0.25 in) or larger. Types of coarse screens include mechanically and manually cleaned bar screens, including trash racks.
Fine screens are typically used to remove material that may create operation and maintenance problems in downstream processes, particularly in systems that lack primary treatment. Typical opening sizes for fine screens are 1.5 to 6 mm (0.06 to 0.25 in). Very fine screens with openings of 0.2 to 1.5 mm (0.01 to 0.06 in) placed after coarse or fine screens can reduce suspended solids to levels near those achieved by primary clarification.
Three stages to wastewater treatment process
1) Primary treatment
The objective of primary treatment is the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25% to 35% of the incoming biochemical oxygen demand (BOD), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation but colloidal and dissolved constituents are not affected.
2) Secondary treatment
The objective of secondary treatment is to remove the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products.
Several aerobic biological processes are used for secondary treatment. These differ primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter. Common high-rate processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contactors.
In the activated sludge process, the dispersed-growth reactor is an aeration tank or basin containing a suspension of the wastewater and microorganisms, the mixed liquor. The contents of the aeration tank are mixed vigorously by aeration devices which also supply oxygen to the biological suspension. Aeration devices commonly used include submerged diffusers that release compressed air and mechanical surface aerators that introduce air by agitating the liquid surface. Hydraulic retention time in the aeration tanks usually ranges from three to eight hours but can be higher with high BOD wastewaters.
Following the aeration step, the microorganisms are separated from the liquid by sedimentation and the clarified liquid is secondary effluent. A portion of the biological sludge is recycled to the aeration basin as return activated sludge (RAS) to maintain a high mixed-liquor suspended solids (MLSS) level. The remainder is removed as surplus activated sludge (SAS) or otherwise know as waste activated sludge (WAS) from the process and sent to sludge processing to maintain a relatively constant concentration of microorganisms in the system.
3) Tertiary treatment
Tertiary and/or advanced wastewater treatment is used to remove specific wastewater constituents which cannot be removed by secondary treatment. Nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids can be removed using individual treatment processes. However, advanced treatment processes are sometimes combined with primary or secondary treatment (e.g., chemical addition to primary clarifiers or aeration basins to remove phosphorus) or used in place of secondary treatment (e.g., overland flow treatment of primary effluent).
4 advanced waste water solutions
Multiple advanced treatment solutions are available, driven by the need for improved operational cost (OPEX), smaller plant footprints and more stringent regulations governing discharge. Below we have highlighted four key waste water solutions.
1) Moving Bed Biofilm Reaction (MBBR) Technology
MBBR technology employs thousands of polyethylene biofilm carriers operating in mixed motion within an aerated wastewater treatment basin. Each individual biocarrier increases productivity through providing protected surface area to support the growth of heterotrophic and autotrophic bacteria within its cells. It is this high-density population of bacteria that achieves high-rate biodegradation within the system. MBBR processes can self-maintain an optimum level of productive biofilm which, when attached to the mobile biocarriers within the system automatically responds to load fluctuations, according to Headworks International.
2) Membrane Bioreactors (MBR)
Membrane bioreactor’ (MBR) is generally a term used to define wastewater treatment processes where a perm-selective membrane, eg microfiltration or ultrafiltration, is integrated with a biological process − specifically a suspended growth bioreactor, according the MBR site. MBRs differ from ‘polishing’ processes where the membrane is employed as a discrete tertiary treatment step with no return of the active biomass to the biological process. Almost all commercial MBR processes available today use the membrane as a filter, rejecting the solid materials which are developed by the biological process, resulting in a clarified and disinfected product effluent.
3) Membrane aerated biofilm reactor (MABR)
MABR systems passively circulate oxygen through a spirally wound membrane at atmospheric pressure. MABR’s self-respiring membrane allows bacteria to consume oxygen more readily for a 90% reduction in energy used for aeration. The membrane surface accumulates a biofilm of bacteria that establishes a simultaneous nitrification-denitrification (SND) process to produce a high-quality, low-nitrogen effluent suitable for reuse in irrigation.
4) Ultraviolet (UV)
For advanced wastewater treatment plants, ultraviolet (UV) technology has been included in the tertiary treatment process. This can allow the wastewater treatment plant to meet even more stringent requirement, in some cases for indirect and direct potable reuse and water reclamation.
The wavelengths of UV light range between 200 and 300 nanometers (billionths of a meter). Special low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the optimal wavelength for disinfection and ozone destruction. Categorised as germicidal, this means they are capable of inactivating microorganisms, such as bacteria, viruses and protozoa. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through UV transparent Teflon tubes.
One notable development to UV systems is the scaling up of light-emitting diode technology, known as UV-LED, with 2018 witnessing a tipping point on power density and purchasing price.
Waste water recovery
Water reuse is a form of wastewater recovery whereby water can be extracted for purposes such as agricultural and golf course irrigation, rather than being discharged to the environment. This ties in with a growing trend to see wastewater treatment plants instead as resource recovery centres. Recovering the water, energy, nutrients and other precious materials embedded in wastewater is a key opportunity, according to the IWA.
Used water is one of the most under-exploited resources available. Water from industrial or domestic use contains energy, water, organics, phosphates, nitrogen, cellulose, rare earths, and other resources. The IWA said technologies are increasingly making resource recovery from wastewater commercially feasible, including bio-gas, fertiliser, paper, metals, plastics and, perhaps most importantly, it is a source of ‘new’ water.
Most water reuse applications prior to the last decade were producing secondary quality water for industrial or agricultural purposes. These will still provide major uses for lower grade reused wastewater. However, for potable and some industrial purposes, a high level of treatment is required.
When addressing the question of why reuse wastewater, one answer is because you've already paid for the treatment so why not make the most of this resource.
Techniques for potable water reuse caninvolve membrane-based techniques such as ultrafiltration (UF) and reverse osmosis (RO), and using ultraviolet (UV) light or ozone for disinfection. Lately, these are finding other applications in industry. Other techniques such as electrodialysis, ceramic membranes and advanced oxidation are also being used in novel ways to enable wastewater reuse.
For potable purposes, the industry has split wastewater reuse into indirect (IPR) and direct (DPR) reuse, the latter requiring much more stringent standards and approvals than the former.
The city-state of Singapore has long been the pathfinder in reusing wastewater for potable purposes - NEWater, as the government terms it. Yet the small cities of Big Spring and Wichita Falls in Texas and the much larger city of San Diego in California will probably be better remembered as ushering in DPR across the world.
The US already hosts a world-leading example of IPR in the Orange County Groundwater Replenishment System in California. Meanwhile, Australia boasts an equally large project, the 232,000 m3/day Western Corridor Recycled Water Project in Queensland. This has three advanced wastewater treatment plants, which contribute reused water to industry and agriculture.