Anaerobic digestion has been shown to be the best technology for treating wastewater from sewage systems globally and is increasingly being used in sewage treatment.
The properties of sludge can be modified during anaerobic digestion and produce biogas as a result of the treatment, as well as several positive effects on sludge management.
Anaerobic digestion promotes stabilized sewage sludge reduction and elimination of harmful pathogens as well as odour emission. But, an excellent process like this can always be made better, and that's what we look at in the rest of this process improvement review article.
Production of Municipal Sewage Sludge
Sewage sludge is the remaining, semi-solid material that is left over after the cleaned-up water is discharged from sewage treatment works. It is the solid material that settles out from municipal or industrial wastewater when the foul sewage is de-gritted and held in large tanks for between half an hour to one and a half hours.
Sludge is potentially hazardous because it contains adsorbed residual organic pollutants from treated wastewater (Bolobajev et al).
Sludge production currently results in serious environmental issues in many developed, and developing, nations. Rapid industrialization, in conjunction with the extensive growth of urban zones, has also raised concerns in relation to sludge disposal (chang et al., 2011b). A wastewater treatment plant is a facility in which a combination of various processes (e. g., physical, chemical and biological) are used to treat industrial wastewater and remove pollutants (Hreiz et al., 2015). The waste residue generated during these treatment processes is known as sludge (Svanstrom et al., 2004, Abellera et al., 2012).
Sewage sludge management is now becoming an ever more serious issue all over the world. Anaerobic digestion is a simple and well-studied process capable of biologically converting the chemical energy of sewage sludge into methane-rich biogas, as a carbon-neutral alternative to fossil fuels whilst destroying pathogens and removing odours.
Hydrolysis is the rate-limiting step because of the sewage sludge complex floc structure (such as extracellular polymeric substances) and hard cell wall.
To accelerate the rate-limiting hydrolysis and improve the efficiency of anaerobic digestion, various pretreatment technologies have been developed.
This article presents an up-to-date review of recent research achievements in the pretreatment technologies used for improving biogas production including mechanical (ultrasonic, microwave, electrokinetic and high-pressure homogenization), thermal, chemical (acidic, alkali, ozonation, Fenton and Fe(ii)-activated persulfate oxidation), and biological options (temperature-phased anaerobic digestion and microbial electrolysis cell).
Most Sewage Sludge Produced in the UK is Treated by Anaerobic Digesters
Over 80% of sewage sludge produced in the UK is treated by anaerobic digesters. The biogas produced is a renewable energy source and can be used to generate electricity and heat or converted into biomethane for grid injection, whilst the biosolids produced are generally dewatered and applied to land.
Operators and managers are faced with the challenges of increasing energy production whilst minimising the costs associated with digestate disposal/recycling at anaerobic digester plants. There are many factors that influence the financial balance. With the advent of 2022 and a competitive bioresources market delivering upon innovation and efficiency, now more than ever it is vital to understand revenue and cost drivers at the site and business levels.
Anaerobic digestion (ad) of sewage sludge is one of the most efficient, effective, and environmentally sustainable remediation techniques; however, the presence of complex floc structures, hard cell walls, and large amounts of molecular organic matter in the sludge hinder ad hydrolysis.
Consequently, sewage sludge pretreatment is a prerequisite to accelerate hydrolysis and improve ad efficiency. This review focuses on pretreatment techniques for improving sewage sludge ad, which includes mechanical, chemical, thermal, and biological processes. The various pretreatment process effects are discussed in terms of advantages and disadvantages, including their effectiveness, and recent achievements are reviewed for improved biogas production.
A paper outlines a multi-objective, integrated approach to analyze various possibilities for increasing the energy efficiency of the largest Italian wastewater treatment plant (WWTP) at Castiglione Torinese. In this approach, wastewater and sludge treatment units are thoroughly investigated to find potential ways for improving the energy efficiency of the system.
Firstly, a multi-step simulation-based methodology is proposed to make a full link between treatment processes and the energy demand and production. Further, a scenario-based optimization approach is proposed to find the nondominated and optimized performance of the WWTP.
The results prove a potential to save up to 5000 MWh of the annual energy consumption of the plant, in addition, to improving the effluent quality through operational changes only.
The Obvious Choice to Improve Biogas Yields – Co-digestion
The co-digestion of ss, cm and ms significantly enhanced ch4 productivity compared to sewage sludge digestion. In summary, anaerobic digestion using sewage sludge as a mono-substrate is not very stable, a lower biogas yield occurs when sewage sludge is digested alone without the addition of a higher calorie organic waste such as food waste.
This is partially related to its lower c/n ratio which can be as low as 5. 30 which is much lower than the data obtained from co-digestion. Undoubtedly, the addition of a higher calorific organic waste the WwTW sludge digesters efficiently enhanced the conversion of organics to methane (biogas). The likely mechanisms can be summarized as follows:
- the sufficient hydrophilic organics supplied
- the buffering capacity originating from the added co-digestate, and
- the rebalancing of C/N and a decrease in ammonia toxicity.
Sequential Ultrasound and Low-temperature (55°C) Thermal Pretreatment
Ultrasound pretreatment provides little trouble in its use and does well to raise biogas yields. A good pairing can be combining it with low-temperature (55°c) thermal pretreatment.
The influence of sequential ultrasound and low-temperature (55°c) thermal pretreatment on sewage sludge solubilization, enzyme activity and anaerobic digestion was assessed in a recent paper.
The pre-treatment leads to significant increases in the soluble concentrations of carbohydrates and proteins, respectively, and higher enzymatic activities in the soluble phase of the sludge. Optimal conditions for chemical oxygen demand solubilization were determined in a recent paper at 59. 3kg/l total solids (TS) concentration, 30,500kj/kg TS specific energy and 13h thermal treatment time using response surface methodology. The methane yield after pre-treatment increased up to 50% compared with the raw sewage sludge.
Electric fields are used for a variety of processes in modern biotechnology. Electrokinetic disintegration is mainly used for sewage sludge treatment. The main inhibiting factor for good anaerobic digestion of sewage sludge is the presence of flocs and aggregates, which are formed by negatively charged molecules on microbial extracellular polymeric substances forming ionic bonds with cations (Tyagi and lo, 2011; Higgins and Novak, 1997).
The application of an electrical field to sewage sludge disrupts these ionic bonds and thus breaks apart the flocs (Tyagi and lo, 2011). It is also likely electric fields disrupt microbial cells by changing the charge of the cell membranes. It is not clear what effect if any, this treatment has on lignocellulosic material.
Biogas is generated from sources, such as municipal organic waste, sewage sludge, muck or manure.
Ultrasonication improves the digestibility of such organic material leading to more biogas and less residual sludge.
Biogas is a byproduct of the decomposition of organic matter by anaerobic or aerobic bacteria. It consists primarily of methane, carbon dioxide and hydrogen sulfide. This makes biogas a renewable alternative to fossil fuels, such as natural gas.
Triple Pretreatment of Waste Activated Sludge with Heat, Alkalinity, and Hydrogen Peroxide
One recent study investigated the possibility of enhancing the anaerobic digestion of sewage sludge with different pre-treatments. It was revealed that the triple pre-treatment of waste activated sludge with heat, alkalinity, and hydrogen peroxide increases soluble fractions of organic matter considerably more than dual and individual pre-treatments.
This led to significantly higher daily biogas and methane production from the anaerobic digestion, as a higher amount of biodegradable organic matter was available to the anaerobic microbial community.
In addition, the effect of input variables and their interactions on methane production were analysed with response surface methodology and optimized input variables were suggested in the end.
Furthermore, harnessing a higher amount of methane in the anaerobic digestion stage decreases methane emission to the atmosphere in the dewatering and landfilling stages and enhances the quality of digested sludge, bringing about an environmentally friendly and economically attractive sewage sludge treatment process.
FNA and Fenton Pre-treatment
A study investigated the feasibility of enhancing anaerobic digestion of mixed primary sludge and waste activated sludge through combined FNA and Fenton pre-treatment. Combined FNA and Fenton pre-treatments were shown to increase soluble fractions of organic matter considerably more than these pre-treatments alone.
This resulted in enhanced biodegradability of organic matter, biogas production, methane production, and COD removal during the anaerobic digestion process. The improved methane production is of paramount importance, not only because higher amounts of renewable energy are obtained from the anaerobic digestion process, but also because of lower methane emission.
Methane is a major greenhouse gas, that is released into the atmosphere. The improved COD of the digested sludge paves the way for having a more integrated and sustainable sludge treatment process. Specifically, this occurs, as sludge transport expenditures are reduced and the digested sludge achieves a higher potential application to agricultural lands.
Options for Dealing with Sewage Sludge
Biorefinery and resource recovery approaches aimed at extracting value-added products from activated sewage sludge have been studied examples of such possible products are, but not limited to:
- biopesticides, proteins) and
- nutrients (nitrogen, phosphorous) from sewage sludge.
Other options in the framework of a circular economy concept, and control options for metal elements and micropollutants exist such as:
- energy recovery routes, such as anaerobic digestion (including pre and intermediate treatments),
- pyrolysis, gasification,
- hydrothermal carbonization (HTC) and
- enhanced digestion using microbial fuel cells,
More study work is likely to continue on many of the above options along with their comparative evaluation, to measure their suitability for different sludge compositions and their resource availability.
Regulation of Land-applied Biosolids
When considering the options for sludge treatment it is vital for designers to consider how local and national regulations apply in the area to protect farmland from the imprudent spreading of substances that may build up over a long period to become toxic.
The current regulatory framework for the exploitation of biosolids was implemented by the US EPA in 1993. Other nations have similar regulations in place.
Designers always need to consider these regulations before declaring the value of any fertilised products. These regulations can have a make-or-break effect on what fertilser products can be sold for land application.
Sewage Sludge Treatment – A Round-up of Wastewater Sludge Treatment Information Available Online
Sewage sludge refers to the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater.
The term septage is also referring to sludge from simple wastewater treatment but is connected to simple on-site sanitation systems, such as septic tanks.
Source: Anaerobic digestion wastewater treatment – via Sewage sludge – Wikipedia
Sewage Sludge Treatment
Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge is mostly water with lesser amounts of solid material removed from liquid sewage.
Defining the terms used:
- Primary sludge includes settleable solids removed during primary treatment in primary clarifiers.
- Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors.
Sludge treatment was [historically] focused on reducing sludge weight and volume to reduce disposal costs, and on reducing potential health risks of disposal options.
Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration.
The choice of a sludge treatment method depends on the volume of sludge generated, and the comparison of treatment costs required for available disposal options.
Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.
Energy may be recovered from sludge through methane gas produced during anaerobic digestion or through incineration of dried sludge.
But incineration has fallen out of favour in the last 10 years due to the fact that the energy yield from incineration is not insufficient to evaporate sludge water content or to power blowers, pumps, or centrifuges required for dewatering.
Coarse primary solids and secondary sewage sludge may include toxic chemicals removed from liquid sewage by sorption onto solid particles in clarifier sludge. Reducing sludge volume has the disadvantage that it may increase the concentration of any toxic chemicals in the sludge.
There is further information available on Wikipedia on biosolids, sewage sludge thickeners, sidestream treatment technologies, and phosphorus recovery, plus anaerobic digestion, and composting of sewage sludge. Via Treatment of SS
Biosolids on the US EPA Website
Sewage sludge treatment standards help protect public health and the environment. Biosolids undergo treatment to make them safe for land application. via Biosolids | US EPA
Wastewater treatment – Sewage Sludge treatment and disposal
The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Sewage sludge is the solid, semisolid, or slurry residual material that is produced as a by-product of wastewater treatment processes. This residue is commonly classified as primary and secondary sludge.
Primary sludge is generated from chemical precipitation, sedimentation, and other primary processes, whereas secondary sludge is the activated waste biomass resulting from biological treatments.
Some sewage plants also receive septage or septic tank solids from household on-site wastewater treatment systems. Quite often the sludges are combined together for further treatment and disposal. via Wastewater treatment
Digestion – Anaerobic Digestion in wastewater treatment
After thickening, the sludge is further treated to make it safer for the environment. The sludge is placed in oxygen-free tanks, called digesters, and heated to at least 95 degrees Fahrenheit for between 15 to 20 days. This stimulates the growth of anaerobic bacteria, which consume organic material in the sludge. Unlike the bacteria in the aeration tanks, these bacteria thrive in an oxygen-free or “anaerobic” environment.
The digestion process stabilises the thickened sludge by converting much of the material into water, carbon dioxide and methane gas. The black sludge that remains after digestion has the consistency of pea soup and has little odour. This is called digested sludge.
Methane gas is often used as an energy source at the City's wastewater treatment plants. The gas may be used in engines to produce electricity or directly drive plant equipment.
Gas is also used in boilers to provide heat for digestion and plant-wide buildings. Currently, DEP and the New York Power Authority (NYPA) have jointly installed fuel cells at four of the City's water pollution control plants; 26th Ward, Red Hook, Oakwood Beach and Hunts Point.
Fuel cells convert the methane gas and carbon dioxide into heat and electricity that is then used to operate the plants. This technology contributes to New York City's efforts to enhance clean air operations at its facilities. There is a significant reduction in air emissions as a result of using fuel cells.
Digester sludge is pumped from sludge storage tanks to a dewatering facility. At some treatment plants, where there are no dewatering facilities on site, the sludge is transported for processing through a pipeline or by a sludge boat to a plant that has a dewatering facility. (See pictures of sludge vessels.) via WWT Process – New York City
It is also increasingly common to use anaerobic digestion processes in industrial wastewater treatment.
The following is the original archived June 2018 version of this post:
Sewage Sludge Treatment by anaerobic digestion is well established, and the challenge now is how to improve Water Industry AD plant performance. The news is that the Global Biogas Expo is where the AD industry will be exhibiting more ways to raise sewage sludge energy and wastewater electricity/ power output than ever before.
In fact, our advice is that all water industry biogas plant professionals responsible for efficient biogas production at municipal wastewater treatment plants should be there.
Biogas Trade Show Press release follows:
A record number of representatives from the water industry are expected at UK AD & World Biogas Expo 2018 at the NEC in Birmingham (July 11th & 12th), the largest global trade show dedicated solely to anaerobic digestion and biogas.
Featuring the very latest in biogas technology, this highly focused Expo, now in its ninth year, presents the water industry with the perfect opportunity to achieve its goal of energy self-sufficiency – with products and systems to increase the generation of renewable energy.
Including the very latest in pumps, mixers, valves, tanks, flares instrumentation and much more, UK AD & World Biogas Expo 2018 is an international who’s who under one roof. It gives water companies and contractors a hands-on look at what would be best for new digesters and how existing sludge tanks can benefit from:
- A boost in biogas yields
- Reduced maintenance
- Reduced carbon emissions
- Reduced energy costs
- Improved health and safety
said Charlotte Morton, Chief Executive of the Anaerobic Digestion and Bioresources Association, which is organising the Expo in partnership with the World Biogas Association.
“But whilst we do of course continue to attract visitors from numerous sectors, it is very encouraging indeed to see such a healthy response from those in the water industry who want to get the very best from their biogas production and biosolids”.
The Expo is expected to attract over 3,000 attendees and more than 200 exhibitors from around the world. There will also be over 100 speakers, including key personnel from Ofwat and heads of bioresources from UK water companies.
This post was first published in June 2018.