Use of Sewage Sludge in Cement Industry

Cities around the world produce huge quantity of municipal wastewater (or sewage) which represents a serious problem due to its high treatment costs and risk to environment, human health and marine life. Sewage generation is bound to increase at rapid rates due to increase in number and size of urban habitats and growing industrialization.

An attractive disposal method for sewage sludge is to use it as alternative fuel source in cement industry. The resultant ash is incorporated in the cement matrix. Infact, several European countries, like Germany and Switzerland, have already started adopting this practice for sewage sludge management. Sewage sludge has relatively high net calorific value of 10-20 MJ/kg as well as lower carbon dioxide emissions factor compared to coal when treated in a cement kiln. Use of sludge in cement kilns can also tackle the problem of safe and eco-friendly disposal of sewage sludge. The cement industry accounts for almost 5 percent of anthropogenic CO2 emissions worldwide. Treating municipal wastes in cement kilns can reduce industry’s reliance on fossil fuels and decrease greenhouse gas emissions.

The use of sewage sludge as alternative fuel in clinker production is one of the most sustainable option for sludge waste management. Due to the high temperature in the kiln the organic content of the sewage sludge will be completely destroyed. The sludge minerals will be bound in the clinker after the burning process. The calorific value of sewage sludge depends on the organic content and on the moisture content of the sludge. Dried sewage sludge with high organic content possesses a high calorific value.  Waste coming out of sewage sludge treatment processes has a minor role as raw material substitute, due to their chemical composition.

The dried municipal sewage sludge has organic material content (ca. 40 – 45 wt %), therefore the use of this alternative fuel in clinker production will save fossil CO2 emissions. According to IPCC default of solid biomass fuel, the dried sewage sludge CO2 emission factor is 110 kg CO2/GJ without consideration of biogenic content. The usage of municipal sewage sludge as fuel supports the saving of fossil fuel emission.

Sludge is usually treated before disposal to reduce water content, fermentation propensity and pathogens by making use of treatment processes like thickening, dewatering, stabilisation, disinfection and thermal drying. The sludge may undergo one or several treatments resulting in a dry solid alternative fuel of a low to medium energy content that can be used in cement industry.

The use of sewage sludge as alternative fuel is a common practice in cement plants around the world, Europe in particular. It could be an attractive business proposition for wastewater treatment plant operators and cement industry to work together to tackle the problem of sewage sludge disposal, and high energy requirements and GHGs emissions from the cement industry.

Ultrasonic Pretreatment in Anaerobic Digestion

Anaerobic digestion process comprises of four major steps – hydrolysis, acidogenesis, acetogenesis and methanogenesis. The biological hydrolysis is the rate limiting step and pretreatment of sludge by chemical, mechanical or thermal disintegration can improve the anaerobic digestion process. Ultrasonic disintegration is a method for breakup of microbial cells to extract intracellular material.

Ultrasound activated sludge disintegration could positively affect sludge anaerobic digestion. Due to sludge disintegration, organic compounds are transferred from the sludge solids into the aqueous phase resulting in an enhanced biodegradability. Therefore disintegration of sewage sludge is a promising method to enhance anaerobic digestion rates and lead to reduce the volume of sludge digesters.

The addition of disintegrated surplus activated sludge and/or foam to the process of sludge anaerobic digestion can lead to markedly better effects of sludge handling at wastewater treatment plants. In the case of disintegrated activated sludge and/or foam addition to the process of anaerobic digestion it is possible to achieve an even twice a higher production of biogas. Here are few examples:

STP Bad Bramstedt, Germany (4.49 MGD)

  • First fundamental study on pilot scale by Technical University of Hamburg-Harburg, 3 years, 1997 – 1999
  • reduction in digestion time from 20 to 4 days without losses in degradation efficiency
  • increase in biogas production by a factor of 4
  • reduction of digested sludge mass of 25%

STP Ahrensburg, Germany (2.64 MGD)

  • Preliminary test on pilot-scale by Technical University of Hamburg-Harburg, 6 months, 1999
  • increase in VS destruction of 20%
  • increase in biogas production of 20%

STP Bamberg, Germany (12.15 MGD)

  • Preliminary full-scale test, 4 months, 2002 2) Full-scale installation since June 2004
  • increase in VS destruction of 30%
  • increase in biogas production of 30%
  • avoided the construction of a new anaerobic digester

STP Freising, Germany (6.87 MGD)

  • Fundamental full-scale study by University of Armed Forces, Munich, 4 months, 2003
  • increase in biogas production of 15%
  • improved sludge dewatering of 10%

STP Meldorf, Germany (1.06 MGD)

  • Preliminary full-scale test, 3 months, 2004 2) Full-scale installation since December 2004
  • increase in VS destruction of 25%
  • increase in biogas production of 25%
  • no foam or filamentous organisms present in the anaerobic sludge digester

STP Ergolz 2, Switzerland (3.43 MGD)

  • Full-scale test, 3 months, 2004
  • increase in VS destruction of 15%
  • increase in biogas production of 25%

STP Beverungen, Germany (2.64 MGD)

  • Full-scale test, 3 months, 2004/2005
  • increase in VS destruction of 25%
  • increase in biogas production of 25%

To sum up, ultrasonication has a positive effect on sludge solubilisation, sludge volume, biogas production, flock size reduction and cells lyses. Ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better degradation of volatile solids and an increased production of biogas.

The use of low power ultrasound in bioreactors may present a significant improvement in cost reduction. Therefore, ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better sludge digestion and efficient recovery of valuables.

Anaerobic Digestion of Tannery Wastes

The conventional leather tanning technology is highly polluting as it produces large amounts of organic and chemical pollutants. Wastes generated by tanneries pose a major challenge to the environment. Anaerobic digestion of tannery wastes is an attractive method to recover energy from tannery wastes.

According to conservative estimates, more than 600,000 tons per year of solid waste are generated worldwide by leather industry and approximately 40–50% of the hides are lost to shavings and trimmings. Everyday a huge quantity of solid waste, including trimmings of finished leather, shaving dusts, hair, fleshing, trimming of raw hides and skins, are being produced from the industries. Chromium, sulphur, oils and noxious gas (methane, ammonia, and hydrogen sulphide) are the elements of liquid, gas and solid waste of tannery industries.

Biogas from Tannery Wastes

Anaerobic digestion (or biomethanation) systems are mature and proven processes that have the potential to convert tannery wastes into energy efficiently, and achieve the goals of pollution prevention/reduction, elimination of uncontrolled methane emissions and odour, recovery of biomass energy potential as biogas, production of stabilized residue for use as low grade fertilizer.

Anaerobic digestion of tannery wastes is an attractive method to recover energy from tannery wastes. This method degrades a substantial part of the organic matter contained in the sludge and tannery solid wastes, generating valuable biogas, contributing to alleviate the environmental problem, giving time to set-up more sustainable treatment and disposal routes. Digested solid waste is biologically stabilized and can be reused in agriculture.

Until now, biogas generation from tannery wastewater was considered that the complexity of the waste water stream originating from tanneries in combination with the presence of chroming would result in the poisoning of the process in a high loaded anaerobic reactor.

When the locally available industrial wastewater treatment plant is not provided by anaerobic digester, a large scale digestion can be planned in regions accommodating a big cluster of tanneries, if there is enough waste to make the facility economically attractive.

In this circumstance, an anaerobic co-digestion plant based on sludge and tanneries may be a recommendable option, which reduces the quantity of landfilled waste and recovers its energy potential. It can also incorporate any other domestic, industrial or agricultural wastes. Chrome-free digested tannery sludge also has a definite value as a fertilizer based on its nutrient content.

Potential Applications of Biogas

Biogas produced in anaerobic digesters consists of methane (50%–80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide.  Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. A combined heat and power plant (CHP) not only generates power but also produces heat for in-house requirements to maintain desired temperature level in the digester during cold season.

CHP systems cover a range of technologies but indicative energy outputs per m3 of biogas are approximately 1.7 kWh electricity and 2.5kWh heat. The combined production of electricity and heat is highly desirable because it displaces non-renewable energy demand elsewhere and therefore reduces the amount of carbon dioxide released into the atmosphere.

AD Plant at ECCO’s Tannery (Netherlands)

A highly advanced wastewater treatment plant and biogas system became fully operational in 2012 at ECCO’s tannery in the Netherlands. A large percentage of the waste is piped directly into the wastewater plant to be converted into biogas. This biogas digester provides a source of renewable fuel and also helps to dispose of waste materials by converting waste from both the leather-making processes, and the wastewater treatment plant, into biogas. All excess organic material from the hides is also converted into biogas.

This project enables ECCO Tannery to reduce waste and to substitute virtually all of its consumption of non-renewable natural gas with renewable biogas. The aim is to use more than 40% of the total tannery waste and replace up to 60% of the total natural gas consumption with biogas.