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Sludge treatment facilities have been using the process of anaerobic digestion for over a century to treat, reduce, and make use of sewage sludge. Below is a primer on anaerobic digestion and its products and benefits, an overview of the process and types of digesters, as well as an introduction to its relevance in the wastewater and sludge industry.

What is Anaerobic Digestion?

Anaerobic Digestion (AD) is a set of biochemical steps where microorganisms break down organic matter such as sewage sludge, manure, and food waste in the absence of oxygen (hence the word &#;anaerobic&#;), primarily producing gases such as methane and carbon dioxide, as well as the organic wet mixture or residue called digestate. Anaerobic digestion is used to treat or stabilise food and other organic wastes, reduce greenhouse gas emissions of otherwise landfilled waste, and extract renewable energy in the form of biogas.

The anaerobic digestion process is utilised by a range of industries, including the agriculture industry for processing manure, energy crops, and agro-industrial waste; the food and manufacturing industries for food processing waste, slaughterhouse waste, pulp and paper liquors, and biochemical waste; and the waste and wastewater industries for municipal organic waste and sewage sludge treatment or management.

It is important to note that there are other stabilisation processes for sludge besides anaerobic digestion, such as alkaline stabilisation (typically the addition of lime), aerobic digestion, composting, and autothermal thermophilic digestion. Anaerobic digestion is, however, seen to be one of the most sustainable options in that it produces renewable energy and lessens sludge or organics volume.

 

The two main by-products of the anaerobic digestion of organic waste can be used in a variety of ways:

Biogas

Biogas is mostly methane and carbon dioxide with small amounts of other gases and water vapor. It can be refined or purified to just biomethane to increase its commercial value. Biogas is considered as a renewable energy source and can be used as fuel or to produce heat and/or electricity. For the sewage industry, the biogas produced from sludge can be used to help offset the energy costs of a wastewater treatment . 

Digestate

 

In the wastewater or sewage industry, digestate or digested sludge is often referred to as &#;biosolids,&#; and its management represents a large percentage of a wastewater treatment plant&#;s operational expenditures, often at about 40%.

What is an anaerobic digester?

An anaerobic digester is the main component of an anaerobic digestion system (also called biogas systems), it is a construction where the anaerobic digestion takes place. These systems can be made using various configurations and different types of equipment, based on the type of feedstock to be treated, the space available, and the final products desired.

Types of Digesters

There are various types of anaerobic digesters used across industries and can be classified based mainly on two things: the engineering design of the reactor and the design of the digester tank or vessel.

There are various types of anaerobic digesters used across industries and can be classified based mainly on two things: the engineering design of the reactor and the design of the digester tank or vessel.

Digester classifications based on reactor engineering:

• mixed digesters
• non-mixed digesters
• continuous flow digesters
• batch digesters
• sequenced batch digesters
• plug flow digesters
• one-stage/single stage digesters
• multi-stage digesters

Digesters such as the complete mixed digester, plug flow digester, and the mixed plug flow digester, are often used in farms.

Digester classifications based on digester tank or vessel design

• pancake digesters
• dome digesters
• lagoon digesters
• cylindrical digesters
• pipe-shaped digesters
• egg-shaped digesters

In the sewage sludge industry specifically, digester tank designs tend today to be cylindrical, with egg-shaped digesters becoming more popular in Europe and lately in the United States.

Digesters can also be classified by whether they treat a low versus high proportion of solids, or whether they operate in mesophilic or thermophilic temperature conditions. The mesophilic temperature range of 30 to 38°C  is most commonly used and caters to &#;mesophilic&#; bacteria which thrive in such conditions, while &#;thermophilic&#; bacteria prefer the temperature range of 50-57°C.

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A cylindrical anaerobic digester at a co-digestion plant in Norway

It is important to note that each type of digester has its own pros and cons, and that the nature and quantity of the material to be digested, as well as the supporting infrastructure, are essential in choosing the digester best suited for a facility. Some digesters need hardly any monitoring, such as lagoons and dome digesters. Others will need more sophisticated monitoring equipment, such as temperature transmittors and more high-tech monitors such as those for volatile fatty acids (VFA), online dry solids (DS), and alkalinity.

What are the steps involved in Anaerobic Digestion?

There are four main steps that take place during the anaerobic digestion process, these are hydrolysis, acidogenesis, acetogenesis, and methanogenesis, all carried out by a diverse microbial community in the absence of oxygen. Though these are the main biochemical reactions that happen within a digester, it must be noted that there are other biochemical reactions occurring in the digester not discussed below.

• Step 1: Hydrolysis &#; the organic feedstock contains compounds that needs to be accessed/broken down before it becomes available as feed for the microorganisms. Complex polymers such as proteins, carbohydrates (polysaccharides), and lipids must first be broken down by into their simpler forms. This &#;breakdown&#; of polymers within a digester tank typically happens via enzymes called hydrolases, secreted by hydrolytic bacteria in the digester.

  Hydrolysis is often referred to as the rate-determining step of anaerobic digestion, meaning it is the slowest step and plays a big part in determining the length of time that the feedstock will stay in a digester. This is why pretreatment methods for anaerobic digestion, such as thermal treatment, focus on optimising this step (see advanced anaerobic digestion section below).

  The steps acidogenesis and acetogenesis happen next simultaneously.

• Step 2: Acidogenesis (fermentation) - In this step, acidogenic or fermentative bacteria present in the digester absorb some of the products of hydrolysis, making intermediate volatile fatty acids or VFAs (also called short-chain volatile organic acids) such as propionate, butyrate, and alcohols. 

  Feedstocks high in protein, such as sewage sludge, also produce plenty of ammonia from the breakdown of amino acids, which is known to make anaerobic digestion more difficult. Besides ammonia, carbon dioxide and other gases such as hydrogen sulfide may be produced.

• Step 3: Acetogenesis - Acetate is formed from the short-chain/volatile fatty acids in acidogenesis, along with hydrogen and carbon dioxide.

• Step 4: Methanogenesis - Methanogenic microorganisms in the digester consume the accessible intermediates produced in acidogenesis and acetogenesis (acetate, hydrogen and carbon-dioxide) to produce methane. This mainly happens via two pathways: acetoclastic methanogenesis and hydrogenotrophic methanogenesis. Besides methane, carbon dioxide in the second most abundant gas produced in this final step.

How is Anaerobic Digestion used in sewage sludge treatment?

Wastewater treatment plants that treat sewage sludge can use anaerobic digestion as a stabilisation or treatment method for sludge. A stabilisation process primarily reduces odour and the decay of sludge while lowering the number of harmful microorganisms. Anaerobic digestion, in addition, lessens the volume of biosolids and gains biogas for the plant. Reduction of final sludge or biosolids occurs because in the sludge are turned into biogas.

Where in the treatment flow is anaerobic digestion placed?

Sludge is introduced to the anaerobic digestion system after primary and secondary wastewater treatment (or the activated sludge process) and thickening. Once anaerobically digested, the sludge (at times referred to as biosolids or digestate by this point) is usually dewatered before the final handling method.

What are the benefits and limitations of Anaerobic Digestion for Sewage Sludge Management?

Anaerobic digestion has multiple benefits. It helps many wastewater utilities or municipalities to achieve the following, but with several limitations.

• Anaerobic digestion reduces pathogens (parasites, viruses, etc.) in sludge. Pathogen reduction may be a priority order to meet regulatory requirements for the final handling method chosen. For example, anaerobic digestion in some countries is sufficient to allow the use of biosolids as agricultural fertiliser, soil conditioner, soil amendment or other soil products. This makes use of the inherent Nitrogen, Phosphorus and other organic material in the material.

  Not all anaerobic digestion systems, however, especially when used without additional treatment methods,  can reduce pathogens sufficiently so that the harmful bacteria do not regrow in the final biosolids product.

• Anaerobic digestion produces biogas that can normally be used to provide heat or electricity through further processing. It also decreases the odour of the biosolids produced.

  Nonetheless, biogas extraction in today&#;s anaerobic digestion systems is highly inefficient. Plenty of energy can remain trapped in the final product when the breakdown of the organic material in the digester is not optimised. This means that methane and carbon dioxide that could otherwise be captured during the process can escape the biosolids material later on when there is further degradation. If this happens in open air (such as a landfill), or in an incineration facility, then the emissions add to the climate problem. The biosolids product also retains sufficiently foul odour to make handling the material unpleasant.

• Anaerobic digestion lessens final biosolids volumes, affecting the costs of transport/further handling.

  Unfortunately though, most anaerobic digestion systems without additional treatment still produce copious volumes of biosolids because plenty of water is trapped within the material even after final dewatering. Learn about sludge dewatering here.

Wastewater treatment facilities worldwide with enough digestion capacity also use anaerobic digestion to digest a combination of sewage sludge and other organics. This is called &#;co-digestion&#;. Sewage sludge can be co-digested with fats, oil, and grease (FOG) &#; a common high-strength liquid organic waste produced by the food processing industry, and with other organic solid wastes such as household or industry food . Co-digestion offers the extra advantage of using food or organic waste as an energy resource without needing to build extensive infrastructure to handle a separate waste stream.

What is Advanced Anaerobic Digestion?

Advanced Anaerobic Digestion (AAD) is a term used to describe the anaerobic digestion when it is modified to create higher quality biosolids (often referred to as Class A by US Environmental Protection Agency standards) and more biogas through higher volatile solids reduction. Modifications that achieve AAD include thermophilic anaerobic digestion, staged thermophilic anaerobic digestion, staged mesophilic anaerobic digestion, acid/gas phased anaerobic digestion, and temperature-phased anaerobic digestion ().

There are also pretreatments that achieve advanced anaerobic digestion. They are classified as either thermal, physical, chemical, or electric pretreatments. Among these, thermal hydrolysis as pretreatment for mesophilic anaerobic digestion stands out due to its growing use in plants around the globe.

Remember that hydrolysis is actually the first step of the actual anaerobic digestion process. By optimising and expediting this step in a pretreatment system, the benefits of anaerobic digestion are boosted, with additional advantages. Thermal hydrolysis, for example, provides up to 50% more biogas, up to 50% more biosolids reduction, and increased digestion capacity, compared to conventional digestion, among other benefits. It is a method used in over 130 plants to date and is already used to pretreat over 50% of the United Kingdom&#;s wastewater sludge before anaerobic digestion. Learn more about thermal hydrolysis here

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