Environmental Engineering Reference
In-Depth Information
Different types of reactors have been constructed and are operational, but the design
approaches could be broadly classified, based on the different stages involved in the
process:
• One-stage reactors provide the conditions for the anaerobic digestion to take place in
one reactor volume, together with a partial thickening of the stabilized sludge and the
supernatant removal.
• Two-stage reactors consist of two separate reactors, working in sequence, where the
first stage is a completely mixed reactor, with intensified conditions for the
fermentation process to take place, followed by a second reactor, which has the main
function to thicken the sludge and provides conditions for the methanogenesis to be
finalized. This configuration is the most widely applied in the practice, specifically for
medium to large treatment plants.
• The latest developments with respect to the research and investigation of the process
suggest multystage reactors, which provide separate compartments for the different
stages of the fermentation process. This type of configuration has not found a wide
practical application yet.
2.2.2 Sludge disposal options
Disposal options for sewage sludge are land application and disposal, incineration and
ocean disposal (USEPA 1995, Attewell 1993, Pescod 1992). In 1982, sludge production
in the USA was estimated at 7 million tones (dry weight), whereas in the European
Community (EC) it is estimated at 6 million tones (dry weight) of raw sludge and is
expected to increase in the future (Pescod 1992). In 1987, 40% of the 1.2 million tones
dry sludge produced in the United Kingdom were utilized on agricultural land while it
was 30% of 6 million tones in the EC countries (Lotter & Pitman 1997). The same
authors mention that in South Africa, 47% of the sewage sludge is disposed on sacrificial
land. The major sludge disposal options include:
• Beneficial land application - this refers to the use of sewage sludge in agriculture as a
fertilizer and also, as a source of water for irrigation;
• Landfills - refers to the cases when sludge is treated together with solid waste, usually
in wet form. In some cases, sludge is dried during the treatment process and after that
used as inert material to fill natural or artificial depressions;
• Incineration - this practice applies to the case, where land is scarce and climatic
conditions do not allow sludge disposal on land options, and as a result, incineration
plants are needed to burn the sludge;
• Surface disposal - applies to the disposal of sludge in dry or wet form on specifically
designated areas, without the sludge being beneficially used;
• Sea (ocean) disposal - this practice has been applied in the past, but has been recently
restricted else-where, and in the EU it has been banned since 1999, due to
environmental considerations.
Beneficial land application and disposal on landfills are the most popular practices for
sewage sludge disposal. In the EC and USA, these disposal practices contribute 70% and
74% of the total sludge generated respectively (Pescod 1992)
