Treating industrial wastewater: anaerobic digestion comes of age: anaerobic treatment systems offer important advantages over conventionally applied aerobic processes for removing organic pollutants from water-based streams. (Cover Story)
In the absence of molecular oxygen, natural environments depend on the activity of anaerobic microorganisms for the biological degradation of organic substrates. In anaerobic environments where, other than carbon dioxide, no inorganic electron acceptors are present, the final degradation of organic compounds is achieved by their conversion to gaseous methane and carbon dioxide. As discussed here, wastewater-treatment systems that rely on the anaerobic digestion process for the removal of organic pollutants in industrial wastewater streams offer important advantages over conventionally applied aerobic processes.
Discussed in detail below, and summarized in Figure 1, are several of the most compelling operational advantages and sources of cost savings associated with using anaerobic biological-treatment systems.
Despite these demonstrable performance and cost-saving advantages--and the fact that nearly 1,600 commercial installations are currently in operation around the world--anaerobic wastewater treatment still remains something of a mystery to many chemical process operators. Many operators still harbor misconceptions and prejudices about the poor biodegradability and presumed toxicity of chemical and petrochemical wastewater constituents on anaerobic microbes. As a result, it is the opinion of these authors that such systems are not currently being applied as widely as they could be within the global chemical process industries (CPI).
This article aims to address some of the lingering misconceptions associated with anaerobic wastewater treatment. It provides a description of the microbiological basis of anaerobic biodegradation, and defines the boundary conditions required to enable anaerobic treatment.
A brief discussion of the pros and cons of today's leading anaerobic bioreactor designs is also provided, along with some simple calculations and basic guidelines for selecting the best bioreactor design for the waste stream at hand. Finally, the article provides a list of organic compounds that have proven to be degradable using anaerobic digestion, and includes several tables that summarize the current status of commercial-scale, anaerobic-treatment systems throughout the global CPI.
Listed here are some of the leading operational advantages and sources of cost savings associated with using anaerobic biological-treatment systems:
1. Anaerobic treatment is in principle an energy-generating process through the production of byproduct methane-rich biogas. The use of the methane for energy generation elsewhere at the plant allows for conservation of more than 90% of the caloric value of the organic substrates being treated. In addition, modern anaerobic bioreactors do not require large energy input for mechanical mixing (which is required to maintain adequate aeration during aerobic treatment).
By comparison, during aerobic treatment, most of the caloric value of the organic substrates is dissipated as non-recoverable heat. And, aerobic bioreactors require significant amounts of energy for aeration.
2. Typically, anaerobic digestion produces only one-fifth to one-tenth as much biomass per unit of organic substrate converted as comparable aerobic processes. Since the disposal or treatment of waste sludge may account for 50% or more of the total waste-treatment costs, the ability to reduce sludge production is a major advantage of anaerobic treatment.
3. All microorganisms require nutrients, such as nitrogen, phosphorus and sulfur, for growth. However, many chemical and petrochemical wastewater streams contain relatively...