An introduction to sewage sludge and sludge treatment

What is sludge?

Sludge from municipalities (often called sewage sludge) is a byproduct of wastewater treatment. It is normally a mix of organic matter from human waste, food waste particles, microorganisms, trace chemicals and inorganic solids from products and medicine we use, together with water bound to these materials. In the United States, sewage sludge is also called wastewater solids.

After any form of treatment, sludge is often referred to as “biosolids”. In line with this, we can say that sludge treatment turns sludge into biosolids. "Biosolids" are usually either applied on land, incinerated, or landfilled.

Sludge can also come from industrial facilities (food manufacturers, pulp and paper factories, chemical and fuel factories, etc.). Industrial sludge is a byproduct of specific factory processes and differs widely in content in each industry. Certain types of industrial sludge are permitted to enter a municipal sewer network for treatment with municipal sludge or be delivered to a sludge treatment plant to be mixed with sewage sludge for treatment.

What is sewage sludge treatment?

Sludge treatment is an important component of municipal wastewater treatment. The primary objectives of sludge treatment are:

  • to reduce the harmful microorganisms (such as pathogens) in sludge, lessening the health risks for people and environments that come into contact with the material;
  • to stabilise some of the organic matter in sludge that would otherwise naturally turn into harmful gases in the atmosphere;
  • to decrease its final volume, typically reducing the associated handling costs; and
  • to collect products and by-products of the treatment process, which may be used or sold to offset the sludge treatment costs.  

Volume reduction and activities that offset sludge treatment costs are significant at wastewater treatment plants. Even though sludge often only makes up 0.2 - 0.4% of the wastewater flow that a treatment plant receives, its treatment often accounts for up to 50% of the facility's operational expenditures.

Why treat sludge?

Wastewater cannot be treated, and clean water cannot be recovered unless the wastewater solids (sludge) are separated. Once sludge is separated from the sewage, it should not be disposed of in its raw form due to its content that can potentially contaminate the natural environment, food systems or directly affect the health of communities.

The disease-causing and foul-smelling components of raw sewage sludge encouraged developed nations to treat the material before disposal from the late 1800s or early 1900s. Today, however, sludge is increasingly seen as a resource. This is because treating sludge through a process called “anaerobic digestion” allows water companies to recover biogas out of the material and make the biosolids a nutrient-rich soil product. Some go further and retrieve from the sludge minerals such as phosphorus, a non-renewable resource. The value of sludge treatment in many places thus goes beyond hazard management and focuses on resource recovery.

Who treats sludge?

Wastewater treatment, and therefore sludge treatment or handling in municipalities, is typically managed by a water utility company. Water utilities can be private players contracted by the government or public entities. They often take care of the clean water supply for a defined geographical area and collect and treat the resulting wastewater. A water utility typically receives wastewater from a city or town’s sewer network at a wastewater treatment plant. 

Some municipalities have the capacity to create centralised wastewater treatment plants connected to extensive sewerage networks. These tend to have dedicated sludge treatment facilities. Towns and rural areas have smaller wastewater treatment plants that may pool their sludge together in sludge centres for proper and cost-effective sludge treatment.

In most developing and emerging countries, wastewater is unfortunately released into natural environments untreated because water utilities do not have sufficient sewerage infrastructure to collect the wastewater or suitable treatment capacity to separate and treat the sludge it contains.

It is also possible for water utilities to gather sludge from septic tanks, i.e. containers for faecal sludge for houses or establishments that are not connected to sewer lines, also known as decentralised systems. Sludge is then ideally transported for treatment to a nearby wastewater treatment plant or a sludge centre.

How is sludge treated?

There are many ways to treat municipal sludge. Water companies prefer cost-effective processes that allow them to deal with sludge while abiding with local or national regulations.

One of the most economic options chosen by many medium and large wastewater treatment plants is digestion. Other options, such as liming and large-scale composting, also stabilise the organic matter in sludge, but we focus on digestion in this article.

During the first wastewater treatment steps, raw sewage sludge is separated from the wastewater through one or several processes. The resulting sludge moves onto a dedicated stream for treatment. Conventional sludge treatment using digestion typically follows these steps in series: thickening, anaerobic digestion, and dewatering before biogas monetisation and biosolids reuse or disposal.

Sludge thickening

Even though most water is separated at the beginning of wastewater treatment, raw sludge is still physically very liquid. Sludge thickening is a low-energy step that separates more water from the solid components before anaerobic digestion. It reduces the overall sludge volume and increases the proportion of solids in the sludge. Sludge before thickening usually contains around 2-4% solids by weight, and thickening can get this content up to 16-18% solids by weight (also called %DS or dry solids).

There are many methods or equipment to accomplish thickening. The simplest is gravitation or gravity thickening, which uses gravity to settle the sludge’s more solid portion. It usually has a slow stirring mechanism that helps settle the “thickened” sludge at the bottom of the gravity thickener. Other thickening methods include dissolved air flotation, centrifugal thickening, gravity belt thickening, and rotary drum thickening. Each of these technologies has its merits and demerits.


How do we quantify sludge?

Though it is physically very liquid before entering the sludge treatment stream, sludge is often expressed in terms of its solids content.  

Common metrics include tonnes* of “dry solids” (tDS) and “% dry solids (%DS).” 

These units show how much solids by weight is in the sludge. This can be measured with a sample in a laboratory.

Sludge, along with its water content, is often expressed in wet tonnes.

* 1 tonne = 1 metric ton


Sludge digestion

There are two main types of digestion in a municipal sludge treatment plant: anaerobic (without oxygen) and aerobic (with oxygen). Of the two, anaerobic digestion is often more cost-effective and has the economic and environmental advantage of producing biogas and lower biosolids volumes.

Anaerobic digestion (AD) uses an elevated temperature and the naturally occurring microorganisms in sludge to break down and then ferment sludge, reducing pathogens in the material. The temperature can either be mesophilic, taking place optimally between 30-38 °C or thermophilic, optimally at 49-57 °C. Mesophilic anaerobic digestion (MAD) is more common because it uses less energy and is more stable, but it requires longer retention times in the digestion tanks.

Anaerobic digestion is a slow process that takes place in large airtight tanks for about 15 to 30 days. Because long periods are needed within these tanks, they are built with a large footprint to room all the sludge that needs to be digested.

The AD process has four biological steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These steps transform the sugars and complex proteins in the sludge into carbon dioxide, water, and methane but still leaves behind a considerable volume of mostly organic solids. As anaerobic digestion produces biogas, a mixture of mainly methane and carbon dioxide, the remaining sludge weight and volume lessen. The biogas is collected and used as fuel to produce heat, electricity or upgraded to biomethane that can be introduced in the natural gas grid or used as a transportation fuel. Even when flared, burning the biogas is important to avoid large methane emissions in the atmosphere, mitigating climate change.

Cities with wastewater facilities but no digestion process tend to use other methods to stabilise the organic matter in sludge. These methods can also be used in combination with digestion.

  • Composting and adding lime, also called alkaline stabilisation, are widespread treatment processes that allow biosolids to be recycled or reused as soil products, just like digestion.
  • Thermochemical treatment procedures like incineration.
  • Thermal drying, which increases the dry solids (DS) content of sludge far more than thickening or dewatering, but which is also very energy-intensive.

Biosolids dewatering

Anaerobically digested sludge is called biosolids and contains mostly water, with the dry solids content typically 6-12%. Dewatering reduces the water content and is different from thickening in that it requires more energy and produces a drier product.

Belt filter presses (BFP) and centrifuges are common dewatering technologies. Belt filter presses use a series of moving belts designed to squeeze the water out of biosolids to a dry solids (DS) content of about 15 to 30% or more, depending on the type of raw sludge and digestion configuration used. Centrifuges “push” the more solid content of sludge to the walls of a rotating vessel and collect the separated water.

Other dewatering methods and equipment include lagoons, solar drying beds, and different presses such as the screw press, filter press, and rotary press. Of course, raw sludge can also be dewatered directly, especially before incineration or landfilling. The dry solids contents of dewatered sludge vary greatly depending on the nature of the sludge and the methods used.

The separated water retrieved in both thickening and dewatering, known as liquors or reject water in some countries, is usually returned to the beginning or inlet of the wastewater treatment facility. In some cases, it must first undergo special treatment, known as liquor treatment.

How are thickening and dewatering different?

Both are means of reducing the water content of the sludge. Thickening consumes less energy as it only removes some of the free water in sludge through methods like settling or centrifugation.

Dewatering removes water bound to the actual solids. It may use chemical processes, primarily adding compounds (called polymers) that help bind wastewater solids better and therefore separate the water more efficiently.

Developments in sludge treatment

New technologies and equipment optimise or expand the sludge treatment process described above, which has remained mostly unchanged for the past century. Some are modifications of the digestion process, and some are specific to the processing of the biogas received from digestion, while others are focused on recovering nutrients, such as phosphorus.

Good examples of development in the industry are the plants using advanced anaerobic digestion and producing high-quality biosolids. Advanced anaerobic digestion refers basically either to a modification of the conventional anaerobic digestion process or to complementary technology or methods that improve digestion by increasing the sludge’s biodegradability. It allows municipalities to process more sludge and produce smaller (better quality) biosolids volumes while generating more renewable energy.

Examples of advanced digestion options include the following:

  • Thermal Hydrolysis
  • Pasteurisation
  • Thermophilic Anaerobic Digestion (TAD)
  • Enzymic Hydrolysis

The thermal hydrolysis process is becoming a well-known pretreatment option for mesophilic anaerobic digestion with multiple cost and environmental advantages. This process is currently used on more than 40% of the United Kingdom’s sludge and is expanding in the European Union, the United States and many other countries.

What happens after sludge is treated?

Once sludge is treated and deemed biosolids, the water utility companies need to send the biosolids product away from the wastewater treatment plant for disposal or reuse. Biosolids handling typically falls into one of these categories:

  • Agricultural use or land application as soil fertiliser, soil amendment, or as part of a soil product;
  • Storage or surface disposal, i.e. sanitary landfilling, mono-filling, and placement in areas usually far from residents;
  • Incineration and subsequent landfilling or reuse of produced ash;
  • Ocean-dumping or open site dumping is widely banned in many countries to date but may still occur.

Of these options, land application on agricultural or non-agricultural land is considered a mode of beneficial reuse, while the others are often referred to as disposal methods. Land application is currently touted as the most environmentally responsible option. Still, it is a challenge even for many developed countries because of limited nearby land or biosolids components that may not be spread on land in too large amounts, such as heavy metals or microplastics.