Agricultural Ammonia for
Stuck Digesters
FRED COOPER, Superintendent
Sewage Treatment Plant
Pullman, Washington
ERVIN HINDIN, Associate Sanitary Chemist
GILBERT H. DUNSTAN, Head
Sanitary Engineering Section
Division of Industrial Research
Washington State University
Pullman, Washington
INTRODUCTION
Anaerobic digestion is the most widely used process for converting raw
sewage sludge into an innocuous substance. It is of major concern to the treatment plant operator and public health officials when a digester becomes distressed.
In a distressed digester the environment has become adverse to the growth of one
or more types of micro-organisms. Methanogenic bacteria, those responsible for
converting the salts of the volatile fatty acids to methane and carbon dioxide, are
inhibited by such environmental changes as a decrease in pH and alkalinity and an
increase in the free and/or salts of the volatile fatty acids.
A number of methods have been used to correct abnormally low pH and alkalinity, thereby providing a favorable environment for the growth of the methane
bacteria. The addition of chemicals to restore the pH and alkalinity of digesting
sludge is a common day practice. Lime has been used to restore the pH of a distressed digester to neutrality since the 1930's. However much controversy exists
even today concerning the use of this additive. The opponents of liming claim
that lime tends to favor the formation of more volatile acids thereby doing more
harm than good (1,2,3). They have cited a number of cases where this is true.
The proponets contend that judicious use of lime can restore a distressed digester
to normal operation (4, 5,6,7).
In the last 10 yrs, agricultural ammonia (anhydrous ammonia) has found use
in restoring pH and alkalinity of distressed digesters. The proponents of using this
additive contend that ammonia is a more efficient neutralizing agent, providing
a readily available supplementary source of nitrogen for the methanogenic bacteria and aiding in dissolving of the scum layer. In certain areas this type of ammonia is cheaper than lime (8,9). Though there has been an increasing use of
anhydrous ammonia and certain basic ammonium compounds to restore normal
alkalinity and pH to a distressed digester, little information has been reported on
dosage, method of application and effect on digester operation.
PROBLEM
A study, whereby the effect of chemical additives on certain parameters of
digestion and on the process itself could be observed,occurred when the primary
digester of a 2.0 mgd biofiltration treatment plant at Pullman, Washington, began to malfunction. The raw sludge from the primary and secondary clarifiers is
pumped  into  a  fixed  cover  conventional  type  digester having  a capacity of
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