BY THE NUMBERS: PROCESS DATA REVIEW REVEALS THE CAUSES OF DIGESTER INHIBITION

For many years, the behavior of the anaerobic digestion system at the Pembroke (ON) wastewater treatment plant was characteristic of a “stuck ”digester. The pH of the digester was low, in the range of 5.0 to 5.2, with minimal volatile solids reduction and digester gas production. A review of operating data and analytical records for the period 1997 – 1999 was initiated in an attempt to determine the causes of the process failure. Comparison of digester operating parameters with benchmark or guideline values helped to indicate some causative factors, including hydraulic and solids over-loading, and low alkalinity in the source water. Of particular note, however, was the low volatile acids concentration of less than 100 mg/L, expressed as acetic acid, in the digester. Analytical data indicated that while all traditional heavy metal concentrations were well below threshold inhibitory levels, the concentration of aluminum (from phosphorus removal) in the digester was at or above the range in which inhibition could occur. Significantly, the inhibitory action of aluminum is on the acid-forming bacteria, rather than on the methane-formers, which usually are the most sensitive group of bacteria in an anaerobic digester. A number of corrective measures were taken to resurrect the digestion process, including temporary cessation of alum dosage, re-seeding the primary digester with a mix of raw wastewater and secondary digested sludge, addition of alkalinity chemicals and urea (for ammonia), and, later, replacement of over-sized raw sludge pumps and substitution of alum with pre-hydroxylated aluminum sulfate. While the measures caused the digester to operate normally, adverse effects from the corrective measures were felt in the primary clarifiers due to a decline in supernatant quality, and in a decline in dewaterability of the digested solids by the plant's belt filter press. The study has shown that the processes of a wastewater treatment plant do not operate in isolation, and that changes to one process may have unintended and unwanted impacts on other processes.