MadSci Network: Chemistry
Query:

Re: What is produced when cleaners biodegrade?

Date: Tue Apr 6 14:26:05 1999
Posted By: Gregory Earl Webb, Masters in Chemical Engineering specializing in cleaning systems and chemicals.
Area of science: Chemistry
ID: 923277906.Ch
Message:

First lets start with some definitions:
What is Biodegradability?

The dictionary's definition of biodegradable is "capable of being readily decomposed by biological means, especially by bacterial action". In the screen printing industry we are concerned with the decomposition of organic materials such as solvents, surfactant, and ink vehicles (or binders); and there are two methods of measuring decomposition. One is "Biochemical Oxygen Demand" (BOD), which is a measure of the actual oxygen consumed by microorganisms during conversion of the material to carbon dioxide and water. The other is "Chemical Oxygen Demand" (COD), which is a measure of the theoretical amount of oxygen required to convert the material to carbon dioxide and water. (Note: The BOD test utilizes active microorganisms, requires a minimum of 5 days, and is difficult to reproduce. The COD test uses common inorganic chemicals, takes about two hours, and is easily duplicated.)

"Percent Biodegradation" is the ratio of BOD/COD times 100. This value is typically reported for date after 5, 10, 20, and 30 days. Some studies suggest that is 50% or more biodegradation occurs within 20 days, the product may be considered "biodegradable" and would be largely removed from the waste water system. However, there is not unanimous agreement on this definition. Some say that compounds that consume oxygen in any amount, at any rate, will eventually biodegrade, and therefore biodegradability becomes a subjective measure. Different methods of determining percent biodegradation may give different results. In order to make meaningful comparisons, the same test methods and calculations must be used.

It should be noted that each municipal sewer district has their own requirements for waste water effluent; many are quite different. Not every screen reclaimer is faced with waste water regulation, but it is becoming the rule rather than the exception. Screen reclaiming products have been specially formulated to be more biodegradable due to a customer's municipal sewer system requirements. However, these products tend to compromise speed of action and cost effectiveness to achieve the required results. A simple solution is often found in the various press wash products offered by Chemical Consultants which can eliminate much of what presently goes down the sewer.

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Here is at least a partial answer to your question. Most biodegradation information concerns the surfactants found in detergents.

Of the massive amounts of LAS and other surfactants used domestically, it is likely that most end up in wastewater flows. However, overall loadings can only be roughly calculated directly due to inadequate data. Reported influent concentrations of LAS range from 3 mg/L (Brunner et al., 1988), and 8.4 and 9 mg/L (De Henau et al., 1989), up to 21 mg/L (Bevia et al., 1989). At an average of 10 mg/L, the LAS loading to a 100 MGD (million gallon per day) treatment facility would exceed 4 tons (U.S) per day. Similar assumptions (c.f. Brunner et al., 1988) suggest that such a facility would also receive between 0.5 and 1.5 tons of the nonionic surfactant NPnEO per day.

LAS and other common surfactants have been considered to be readily biodegradable by aerobic processes, based on laboratory studies (Swisher, 1970). LAS evidently undergoes nearly complete biodegradation, and 97-99% removal rates have been found in wastewater treatment plants (Brunner et al., 1988; Bevia et al., 1989; De Henau et al., 1989). However, the mass loadings indicated above suggest that even at these removals, appreciable amounts are released to receiving waters. Thus Ventura et al. (1989) identified LAS and a variety of other anionic, cationic and nonionic surfactants in both surface and drinking water extracts.

Alkylphenol ethoxylates such as NPnEO are evidently less biodegradable than LAS with laboratory results ranging from 0-20% based on oxygen uptake (e.g. Swisher, 1970; Steinle, 1964; Pitter, 1968) and a wider range of removals from 0-90% based on specific analyses such as UV and IR spectroscopy (Swisher, 1970). This suggests that only partial degradation occurs, such as conversion from polyethoxylates to nonylphenol diethoxylate (NP2EO), nonylphenol monoethoxylate (NP1EO), and nonylphenol (NP). Mass balances done on treatment plants in Switzerland (Brunner et al., 1988) support this.

The findings of Brunner et al. and others also show that the nearly complete removal of surfactants from treated waters is not entirely due to biodegradation. Brunner et al. indicated 19%, and other studies 27% (Rapaport and Eckhoff, 1990), or even in excess of 50% (Bevia et al., 1989) of the surfactant load into a treatment facility to be associated with suspended solids and thus directed into solids treatment processes via primary sedimentation. Surfactants such as LAS are not biodegraded by either mesophilic or thermophilic anaerobic digestion (McEvoy and Giger, 1985; Swisher, 1987) and thus a large proportion of these materials simply escapes associated with sludge solids.

The resulting concentrations of surfactants in sewage sludges can be substantial. LAS concentrations measured in sludges often make up between 0.5% and 1.5% of the dry solid mass, particularly for anaerobically digested sludges (McEvoy and Giger 1986; De Henau et al. 1989; Holt et al. 1989; Marcomini et al. 1989). Bevia et al. (1989) reported LAS between 2% and 4% of the solids weight. In a study of 29 Swiss treatment plants, LAS concentrations averaged 4.2 and 2.1 grams per kilogram respectively in anaerobic and aerobic sludges. NP exceeded 1 gram per kilogram dry sludge and, in some instances NP1EO and NP2EO exceeded 0.1 gram per kilogram dry sludge (Brunner et al. 1988).

I would suggest searching the epa database for a more complete treatment of this subject.

http://www.epa.gov/

Greg Webb


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