48    TREATMENT OF LOW STRENGTH DOMESTIC
WASTEWATER BY USING UPFLOW ANAEROBIC
SLUDGE BLANKET PROCESS
Ning H. Tang, Professor
Environmental Engineering
University of Puerto Rico
Mayaguez, Puerto Rico 00681
Carlos L. Torres, Environmental Engineer
Puerto Rico Water and Sewer Authority
Ponce, Puerto Rico 00734
Richard E. Speece, Centennial Professor
Environmental Engineering
Vanderbilt University
Nashville, Tennessee 37235
INTRODUCTION
Conventional aerobic activated sludge is commonly used for secondary treatment of municipal
wastewaters. While it is an effective and well tested secondary treatment process, it has two
major drawbacks. First, it is energy intensive, requiring about 40 horsepower per MGD to provide oxygen transfer. Second, like all biological treatment processes, it synthesizes easily
degradable, soluble colloidal organic pollutants which cannot be settled out of the wastewater
into very difficult to degrade bacterial cells which can be flocculated and settled out in the secondary clarifier. Aerobic processes synthesize approximately 50% of the BOD into bacterial cells
while anaerobic processes synthesize only about 5% or 1/10 as much excess sludge. Waste activated sludge is very difficult to dispose of. Waste activated sludge must first be stabilized in a digestion process, then land disposed or incinerated. In conventional municipal anaerobic digesters, waste activated sludge constitutes 75% of the volume of sludge feed and yet only 30%
of the waste activated sludge is biodegraded during the digestion period—with 70% being refractive and added to the volume and mass of final sludge disposal. In summary, the sludge disposal
problem of aerobic activated sludge processes is about ten-fold that of anaerobic treatment
processes.
Anaerobic treatment processes require no energy intensive oxygen transfer, but are more sensitive to temperature than their aerobic counterparts. This has been a barrier to the implementation
of the anaerobic treatment of municipal wastewaters in the Netherlands where winter temperatures hinder the process efficiency. However, in tropical climates, this has not been a barrier. In
Colombia, South America, several large prototype installations of anaerobic treatment processes
for municipal wastewater have proven to yield stable, effective treatment.
The tropical environment of Puerto Rico offers great potential for using anaerobic treatment in
place of conventional, aerobic activated sludge processes in the treatment of its warm, dilute municipal wastewaters. It will minimize the troublesome problem of land disposal of municipal
sludges, achieve secondary effluent standards and not be an energy intensive form of treatment.
When the infrastructure of sewage treatment needs to be improved, anaerobic sewage treatment
may serve as one of the better alternatives.
Anaerobic sewage treatment is a totally enclosed process. It has very little environmental impact on the surrounding areas of the treatment site. However, sometimes its effluent may cause
serious odor problems. There are many small communities in Puerto Rico where the anaerobic
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