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.

Bioenergy in the Middle East

The Middle East region offers tremendous renewable energy potential in the form of solar, wind and bioenergy which has remained unexplored to a great extent. The major biomass producing Middle East countries are Egypt, Algeria, Yemen, Iraq, Syria and Jordan. Traditionally, biomass energy has been widely used in rural areas for domestic purposes in the Middle East. Since most of the region is arid/semi-arid, the biomass energy potential is mainly contributed by municipal solid wastes, agricultural residues and agro-industrial wastes.

Municipal solid wastes represent the best bioenergy resource in the Middle East. The high rate of population growth, urbanization and economic expansion in the region is not only accelerating consumption rates but also accelerating the generation of municipal waste. Bahrain, Saudi Arabia, UAE, Qatar and Kuwait rank in the top-ten worldwide in terms of per capita solid waste generation. The gross urban waste generation quantity from Middle East countries is estimated at more than 150 million tons annually.

In Middle East countries, huge quantity of sewage sludge is produced on daily basis which presents a serious problem due to its high treatment costs and risk to environment and human health. On an average, the rate of wastewater generation is 80-200 litres per person each day and sewage output is rising by 25 percent every year. According to estimates from the Drainage and Irrigation Department of Dubai Municipality, sewage generation in the Dubai increased from 50,000 m3 per day in 1981 to 400,000 m3 per day in 2006.

The food processing industry in Middle East produces a large number of organic residues and by-products that can be used as source of bioenergy. In recent decades, the fast-growing food and beverage processing industry has remarkably increased in importance in major countries of the Middle East.

Since the early 1990s, the increased agricultural output stimulated an increase in fruit and vegetable canning as well as juice, beverage, and oil processing in countries like Egypt, Syria, Lebanon and Saudi Arabia. There are many technologically-advanced dairy products, bakery and oil processing plants in the region.

date-wastes

Date palm biomass is found in large quantities across the Middle East

Agriculture plays an important role in the economies of most of the countries in the Middle East.  The contribution of the agricultural sector to the overall economy varies significantly among countries in the region, ranging, for example, from about 3.2 percent in Saudi Arabia to 13.4 percent in Egypt. Cotton, dates, olives, wheat are some of the prominent crops in the Middle East

Large quantities of crop residues are produced annually in the region, and are vastly underutilised. Current farming practice is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels or thermochemically processed to produce electricity and heat in rural areas.

Energy crops, such as Jatropha, can be successfully grown in arid regions for biodiesel production. Infact, Jatropha is already grown at limited scale in some Middle East countries and tremendous potential exists for its commercial exploitation.

The Middle Eastern countries have strong animal population. The livestock sector, in particular sheep, goats and camels, plays an important role in the national economy of the Middle East countries. Many millions of live ruminants are imported into the Middle Eastern countries each year from around the world. In addition, the region has witnessed very rapid growth in the poultry sector. The biogas potential of animal manure can be harnessed both at small- and community-scale.