ANAEROBIC TREATMENT OF A VARIETY
OF INDUSTRIAL WASTE STREAMS
Meint Olthof, Vice President
William R. Kelly, Project Manager
Gregg Wagner, Chemist
Jan Oleszkiewicz, Consultant
Duncan, Lagnese, and Associates, Inc.
Pittsburgh, Pennsylvania 15237
INTRODUCTION
During the last few years the use of anaerobic treatment as applied to industrial waste streams
has received wide attention. Especially in Europe, a large amount of research and development work
has been done on anaerobic treatment and this has resulted in the successful installation of numerous
anaerobic waste treatment systems.
In general the appeal for anaerobic over aerobic treatment is for the following two reasons:
1. Anaerobic treatment results in substantially less sludge than Ihe aerobic counterpart. The bioconversion
of organics in an anaerobic manner results in much less available energy for bacteria that can be used
for cell reproduction and therefore an organism has to convert more organics in order to form a new
cell. The bulk of Ihe energy available in the organics is lost in ihe release of methane. With aerobic
treatment the sludge yield coefficient can be anywhere from 50 10 100% of [he BOD removed while
with anaerobic treatment typically the amount of sludge formed is 10 lo 20% of ihe amount formed
aerobically. With the new regulations on sludge disposal, especially for industrial sludges, Ihis is a large
incentive lo consider anaerobic treatment.
2. The other big advantage of anaerobic over aerobic treatment is the energy situation. With aerobic treatment, it is necessary to provide 1 to I'/; lb of oxygen for every pound of BOD removed. This results
in a net energy cost of about 4 to 6 cents per pound of BOD removed. With anaerobic treatment, it is
possible to get about 5 cubic feet of methane per pound of BOD removed. This represents an available
energy of about 4,000 to 5,000 BTU per pound of BOD. At a typical value of $5.00 per million BTU,
this represents a net energy yield of about 2 to 3 cents. About 20 to 40% of this energy available will
be used for maintaining the temperature of the anaerobic reactor at 35 C. In a large number of industrial
applications, this energy picture is even more favorable in case the waste stream is hoi or when waste
heat is available in the form of low pressure steam.
Over the years a lot of work has been done on applying anaerobic treatment to the natural first
applicant, i.e., the food industry. However, with time the chemical industry is also becoming more
and more interested in applying anaerobic treatment to a variety of their waste streams. Table I lists
successfully treated waste streams. Some of them are tested on a laboratory scale; some are in operation at a full scale installation. Not too long ago, it was thought that anaerobic treatment would
only be applicable to food industry waste and that the application of this technology to chemical
waste would be a risky venture. Some bad publicity on anaerobic treatment was received as a result
of malfunctions in anaerobic digesters of municipal treatment plants. However, this unfavorable
publicity resulted from the fact that these digesters were not always operated very well and that there
was inadequate mixing in these large batch reactors. By applying the same chemical engineering principles to anaerobic treatment as was done with aerobic treatment, it is possible to obtain highly
efficient anaerobic reactors. In this way, it is possible to separate the hydraulic from the solids retention time (SRT) and therefore, the hydraulic retention time can become a matter of hours while
the SRT can be 20-100 days. This is in contrast to the typical detention times (both hydraulic and
solids) of 10 to 30 days as in the case of the municipal digesters. In this way the anaerobic equivalent
of activated sludge and trickling filters was developed. Another myth that had to be overcome before
anaerobic treatment could be considered for the chemical industry waste was the fact that anaerobic
697