The waste sludge from purification is the fraction of solid matter contained in the waste water that is removed in the purification plants during the various purification treatments. Since they contain a large concentration of organic substances it is possible to carry out biological treatments that generate biological sludge to be treated and disposed of as waste, thus becoming a significant cost for the purification plant operators. It is therefore particularly useful and important to find solutions that allow to minimize their production.
The activated sludge process is by far the most widespread biological process to remove the biodegradable organic substance present in sewage (wastewater).
In the process, the biodegradable organic substance is partly oxidized by the bacterial populations present in the treatment tanks (biological sludge)
and partly transformed into new biomass (bacterial growth, thanks to the energy produced by the oxidation process).
At equilibrium, biomass grown up must correspond to an equal mass that is extracted from the process.
The biomass extracted is called ÔÇťbiological excess sludgeÔÇŁ.
The increasing cost of sludge disposal pushes the search for solutions that allow:
(1) to maximize the recovery of energy and matter from sludge, (2) to minimize the mass produced to be disposed of.
The sludge reduction is consequent to the second alternative and allows to reduce the costs for final disposal of sludge, where it cannot be used conveniently (for instance, in agriculture) or where it is not possible to recover its energy content.
In the PerFORM WATER 2030 project a very promising technology is studied for the minimization of biological sludge, namely ozonolysis.
Ozonolysis is the action that exerts ozone on the biological excess sludge. The name means "lysis" (from the ancient Greek ╬╗¤Ź¤â╬╣¤é, "dissolution, separation")
of the organic substance that constitutes the "sludge". In its application to the activated sludge process, ozonolysis, in addition to "dissolving" part of the organic substance,
acts on bacterial growth with three other different "mechanisms", two direct and one indirect:
1) in the first place, thanks to its bactericidal action, the decay rate increases, ie the mortality rate of the bacterial population;
2) secondly, it forces biomass to use part of the energy produced by oxidation to produce enzymes aimed at protecting the biological cells from the ÔÇťattackÔÇŁ of ozone; as biomass would have used this energy for its growth, the consequence is that less biomass grows to remove the same amount of organic matter;
3) finally, ozone promotes the formation of compact flakes and allows to maintain higher concentrations of "sludge" in the treatment tanks; the longer residence time of the biomass in the oxidation tanks increases the fraction of oxidized organic matter and further reduces the production of excess sludge.
As of today, ozonolysis is applied for the reduction of excess sludge in plants where an ozonation treatment is already present,
for example for the removal of non-biodegradable organic pollutants and/or color (as in the case of discharges from dyeing or printing processes in the textile industry)
or where there is already a pure oxygen process, basic raw material for ozone production.
The application of ozonolysis to the activated sludge process is appropriate where the reduction in the amount of sludge to be disposed of allows a greater saving in the cost of ozone production.
The process allows the manager considerable flexibility. If the cost of sludge disposal increases, the manager can increase the ozone dosage by compensating the higher cost with the higher savings in sludge disposal costs. Conversely, if the cost of sludge disposal is low, it will be necessary to reduce the ozone dosage and save on the cost of ozone, giving up part of the reduction of sludge production.
Introduction to biological sludge on the website of the Australian and New Zealand Biosolid Partnership
Introduction to biological sludge on the website of the EPA (United States Environmental Protection Agency)
A brief historical overview of biological sludge treatments on EJnet.org (a website dedicated to the dissemination of environmental issues)
Informative video "Wastewater Treatment Video 5: Secondary Treatment" with a brief description of the biological processes of waste water treatment by Rural Community Assistance Partnership (RCAP), network of non-profit organizations across the United States
Informative video "Wastewater Treatment Video 6: Solids and Sludge Handling" with a brief description of the biological processes of waste water treatment by Rural Community Assistance Partnership (RCAP), network of non-profit organizations across the United States
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