DETERMINATION OF ALKALINITY REQUIREMENTS
FOR THE ANAEROBIC TREATMENT PROCESS
Alan Li, Process Engineer
Paul M. Sutton, Senior Process Engineer
Dorr-Oliver, Incorporated
Stamford, Connecticut 06904
INTRODUCTION
It has long been recognized that control of pH is essential for the successful operation of the
anaerobic process in wastewater treatment [1]. Methane producing bacteria are extremely sensitive to
the changes in environmental conditions. Any effects on methane bacteria usually result in the accu-
lation of volatile acids and a higher C02 content in the gas, both which tend to decrease the reactor
pH. If the pH is not maintained due to a lack of buffering capacity, the condition will further retard
the methanogenic activity and result in a process upset. Many municipal anaerobic digesters become
"sour" as a result of inadequate pH control.
Reactor pH is usually controlled near neutral by addition of alkalinity or alkalinity-producing
chemicals to the reactors, although other methods such as reduction C02 in the gas phase, have been
proposed [2]. For anaerobic treatment of industrial wastewaters containing little or no alkalinity, the
costs associated with pH control can be very significant. For example approximately 10,900 kg
(24,000 lbs) of sodium bicarbonate are required per day for anaerobically treating 3,780 m3/day (103
gal/day) of wastewater containing no alkalinity in order to maintain 2,000 mg/1 of alkalinity as
CaC03.
Although researchers have completed studies on pH control in anaerobic reactors, no single design
approach has been accepted for estimating the alkalinity requirements. In the past, lime has been
widely used for pH adjustment in the municipal anaerobic digester, and the dosage requirements for
maintaining the pH between 6.5 and 7.0 were based on empirical experience. Addition of 4.5 to 6.8 kg
(10 to 15 pounds) of lime per 1000 population per day, or per 28 m3 (1000 ft3) of digester capacity per
day, was recommended for digester pH control [3,4], Further development in anaerobic process
research considered the volatile acids in the digesters as the major demand for alkalinity, and volatile
acid alkalinity was used to determine the alkalinity requirement for the digester without much success
[5,6].
The concept of having enough bicarbonate alkalinity to maintain C02 equilibrium in the digester
was proposed by Benta and Pomeroy [7] in the 1930's. Murray [8] used the concept to estimate the
alkalinity requirement for the anaerobic treatment process with moderate success. This alkalinity and
C02 equilibrium relation was discussed in great detail by Capri and Marais [9] and Loewenthal and
Marais [10]. However, they all failed to include the alkalinity required for volatile acid neutralization.
The purpose of this paper is to present a rational method for estimating the alkalinity requirements in the anaerobic treatment process. The method is based on the principle that the alkalinity required is mainly for the neutralization of carbonic and volatile acids. The results from a pilot plant
study involving a two phase (acid stage followed by methane stage) anaerobic fluidized bed system
(Anitron System™) were used to test the validity of the model. A process design procedure for pH
control is proposed and illustrated in a design example. Finally, the paper also discusses the selection
of pH chemicals for use in the anaerobic treatment process.
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