An introduction to wastewater treatment

What is wastewater?

Water that has been used for any industrial, commercial, or residential purpose becomes wastewater. Municipal wastewater (sewage), specifically, is all water and waste collected from drains, sinks, toilets, and gutters, and may include rain and stormwater. Municipal wastewater comprises water and particles that typically come from faeces, food waste, chemicals from personal hygiene products, cleaning agents, cosmetics and medicines, and other products. These solids or particles are either dissolved, suspended, or floating in the wastewater. Through treatment, these are separated or filtered out as sludge.

Industrial facilities also generate considerable amounts of wastewater that have chemicals or contents specific to their manufacturing process. This is called industrial wastewater.

Two terms that refer to wastewater depending on its location in a treatment process are influent and effluent. Influent basically refers to wastewater that flows into a treatment unit, while the effluent is the wastewater that flows out of the same unit after treatment.

What is wastewater treatment?

Wastewater treatment refers to the processes that wastewater undergoes to reach a specific standard or discharge quality, often set by local or national regulators.

In cities and towns with centralised sewer networks, this is usually done at a wastewater treatment plant, also called a sewage or wastewater treatment works, sewage treatment plant, or water reclamation facility. Here, wastewater goes through several steps before reuse or discharge into a water body.

In places without sewer networks, buildings or residences can have septic tanks to accomplish wastewater treatment (decentralised systems). A septic unit receives wastewater and allows it to form three layers: floating solids and scum on top, wastewater, and sludge at the bottom. The wastewater is separated and released into the soil to be filtered naturally. The septic wastewater that enters the soil still contains harmful components, e.g., hormones, pathogens, trace metals. These can potentially contaminate groundwater systems and pose a danger to the community and environmental health. The sludge that remains in the septic tank is pumped out for sludge treatment or disposal every few years.

Wastewater Treatment Plants and Septic Tanks

Industrial wastewater, depending on its characteristics, may be allowed in some areas to be treated together with municipal wastewater. Other areas, however, require that industrial wastewater be treated at a dedicated treatment plant separate from municipal sewage treatment works.

Why treat wastewater?

Regulatory bodies that require wastewater treatment do so because discharging untreated wastewater into nature can negatively affect the health of communities and the environment.

Pathogens or dangerous bacteria, heavy metals, and other chemicals exist in wastewater along with good microorganisms and valuable nutrients. These harmful substances can cause disease in communities that use wastewater-laden rivers or streams for bathing, drinking, or cooking. Drinking from such sources puts people at risk of diarrhoea, cholera, dysentery, or typhoid. Contracting such diseases on a large scale affects the development of local economies and helps poverty to persist.

Microorganisms and chemicals in wastewater at specific concentrations can also greatly disturb or damage natural ecosystems. Discharging raw sewage can cause pollution in water bodies and on land where the infected water is used, such as in irrigation. Some wastewater discharges with high concentrations of nutrients such as phosphorus and nitrogen, for example, can help cause toxic algal blooms in water bodies through a process called eutrophication. This growth of algae on the surface of water systems can create dead zones as it blocks sunlight and helps deplete the aquatic ecosystem of oxygen.

The United Nations in 2017 estimated that over 80% of global wastewater is released into the environment untreated. This means that though discharging untreated wastewater into natural environments has been proven harmful, many cities and towns still lack strict wastewater and sludge regulations and wastewater treatment infrastructure (sewer networks, treatment plants).

How is wastewater treated?

Many technologies are available for treating wastewater at a centralised facility. Technologies and methods are often chosen based on their suitability in treating the specific wastewater produced by a community or a factory to meet the required thresholds for effluent quality. The cost and awareness of available technologies are also important factors.

Many conventional wastewater treatment plants use the following series of treatments: screening, primary treatment, secondary or biological treatment, polishing (disinfection and filtration), and sludge treatment. The typical steps at such plants are described below.

Screening

The purpose of screening is to remove specific solid items from the wastewater that can either clog pipes and pumps or cause equipment to wear at a faster rate.

A bar or mesh screen is usually used to separate larger items like plastic bags, plants, and textiles, followed by a grit removal chamber for the removal of stones, gravel, human hair, and sand. Once such items are removed, they are usually trucked away to a solid waste landfill or incinerated.

Primary Treatment

The purpose of primary treatment is to remove the settleable and floating solids, traditionally through mechanical means. It can also be aided by chemicals, i.e. polymers that help bind the organic wastewater solids together, making them easier to remove.

The principles that govern primary treatment are filtration, gravity, and sedimentation. In a conventional wastewater treatment plant, gravity and sedimentation are utilised to remove the suspended solids. Denser wastewater solids settle down to the bottom of a basin or tank while lighter particles float to the top. These solids are then skimmed or raked and separated from the remaining wastewater. This removed sludge is called raw or primary sludge, which goes onto sludge treatment while the effluent moves onto secondary treatment.

Traditional technologies or equipment that accomplish conventional primary treatment include primary clarifiers, sedimentation basins, settling basins, or primary settling tanks. More recent technologies include the lamella separator or clarifier, which also works with sedimentation; rotating belt filters (RBF), which additionally incorporate filtration; and nano-bubbles or micro-bubbles to float suspended solids more easily, as in the process of dissolved air flotation or induced gas flotation.

Secondary or Biological Treatment

Secondary or biological treatment removes the carbon, nitrogen, and phosphorus present in wastewater in either a dissolved or particulate form. Conventional biological treatment typically uses the process of aeration to grow the microorganisms in the wastewater, followed by secondary sedimentation or a clarification step. This process was also called the activated sludge process during its inception over a century ago. Today, the term “activated sludge process” can be used to describe most biological processes in wastewater treatment that use air or oxygen to create a more easily separable mass of microorganisms and other wastewater solids.

Secondary treatment based on the activated sludge process works like so: Microorganisms naturally present (or seeded) in the wastewater are either exposed to atmospheric air or pumped with air or oxygen. This is typically done in an aeration tank or basin. The oxygen helps these microorganisms to reproduce and consume certain organic solids in the wastewater (fats, sugars, and other biodegradable components in food and human waste). Through reproduction, the microorganisms and other organic solids become denser and form a biological floc. It can then settle out or float more easily and is separated as secondary or “waste activated” (WAS) sludge in the secondary sedimentation or clarification step. WAS or secondary sludge goes onto sludge treatment, while the effluent proceeds to polishing or disinfection.

Technologies used for biological treatment, besides aeration tanks and basins, include membrane bio-reactors (MBR), trickling filter bed filters, moving bed biofilm reactors (MBBR), integrated fixed film reactors, biological aerated filters (BAF), and sequencing batch reactors (SBR), to name a few. The secondary sedimentation or clarification step can be done with secondary clarifiers, dissolved air flotation tanks, and rotating belt filters. Sequencing batch reactors cover all steps in the same unit.

Advanced technologies for wastewater treatment

Disinfection and Filtration

The effluent of secondary treatment is usually free from dense solids or sludge as it goes onto the disinfection and filtration steps, sometimes referred to as “polishing” or “tertiary treatment”. Disinfection and filtration aim to ensure that the wastewater does not have harmful concentrations of disease-causing microorganisms or toxic compounds before it is either released into the environment or reused by a municipality for applications such as agriculture, toilet flushing, cooling, etc.

Several processes and technologies can accomplish disinfection. With ozonation and chlorination, chemical compounds and reactions kill microorganisms and form insoluble compounds that can be filtered out. In the case of ultraviolet light, radiation is used to destroy the genetic structure of microorganisms. Filtration uses various filtering mediums to capture insoluble compounds. Filtration technologies include sand filters, strainers, rapid gravity and pressure filters, granular media filters, and membrane filters.

Measurement of discharge or effluent quality

Before release into the environment or reuse as clean water, the treated effluent must be sampled to determine if it meets the discharge standards set by local or national regulations. Samples are also normally taken at the beginning of the wastewater treatment process to understand the characteristics of a municipality’s sewage and compare the levels of microorganisms and other particles before and after treatment. Water quality indicators typically include Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), phosphorus, nitrogen, and faecal coliform or bacteria.

What about sludge treatment?

Sludge is rich with nutrients and resources, but like wastewater before disinfection and filtration, it contains harmful microorganisms and other particles that can disrupt natural environments. It is primarily treated to reduce such pathogens and compounds before it is stored or reused.

Sludge treatment also serves to stabilise or reduce the organic matter in sludge that naturally turns into gases (methane and carbon dioxide) in the atmosphere; to decrease its final volume, typically reducing the associated handling costs for utilities; and to collect products and by-products of the treatment process, which may be used or sold to offset some of the costs of sludge treatment.  

Ideally, the sludge from primary and secondary treatment goes through a modern sludge treatment process to recover nutrients and biogas, reduce pathogens, and lower biosolids volumes. In many cases, however, sludge is only dewatered (and possibly limed) prior to disposal or land application. Some wastewater treatment plants are able to use sludge to produce high-quality fertiliser and soil products (called Class A biosolids in the United States).