Effect of Heavy Metals on Anaerobic
Digestion and the Role of Regulatory
Agency when Municipal Sewage Treatment
Plant is Upset
TERRY M. REGAN, Acting Director
Department of Water Pollution Control
City of Lexington
Lexington, Kentucky
MERCER M. PETERS, Chief Chemist
Water Pollution Control Commission
Frankfort, Kentucky
INTRODUCTION
The main Lexington Waste Water Treatment Plant is a 12 MGD Kraus Modified
Activated Sludge Plant with dual aeration. The digestion facilities consist of two
primary digesters with a capacity of 1 MG each and equipped with PFT floating
covers, external heat exchangers and Perth gas recirculation equipment. In addition
to the primary digesters, there are four secondary digesters with a capacity of
approximately 300,000 gal each and equipped with PFT floating covers and stand by
hot water heating coils. At the time of the failure, the average daily flow at this plant
was some 12 MGD with a daily withdrawal of some 75,000 gal of raw sludge, having
a total solids content of some six to eight per cent from the primary settling tanks to
the primary digesters daily.
PLANT OPERATION
Sludge is pumped from the primary settling tanks every two hr to the primary
digesters. The primary digesters are used alternately and, therefore, receive sludge on
a four hr alternating schedule. At the time of the failure, there was a 20 to 25 day
retention time in the primary digesters and a 10 to 15 day retention in the secondary
digesters. The primary digesters produce over 200,000 cu ft of gas a day. This gas is
the primary fuel for three blower engines, all heat exchanger units and the heating
system for the three largest building complexes in the plant. This gas represents 85
per cent of the plant's fuel requirements or, in dollars, some $36,000 per year. As
important as this gas production is, it is not as important as the production of usable
supernatant or digester liquor. The gas represents cost savings but the supernatant is
essential for the operation of a Kraus modified plant. Without supernatant the Kraus
plant is converted to a conventional activated sludge plant with a resulting loss of
some 30 to 40 per cent of capacity.
The city of Lexington, with a population of some 80,000 (in 1967), is located
atop a geological dome some 15 miles from the nearest large body of water (The
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