Process Problems - Lincoln Treatment Plant

The District faced 4 difficult problems during the early years of operations:

a. Biosolids Disposal
b. Composting Odors
c. Excessive weight on RBC Shafts
d. CSO discharges

The District lime stabilized and land applied biosolids from 1982 to 1988 on silage corn fields of local dairy farmers. Permitting sites was difficult due to public opposition and incorporation of the biosolids into the soil was necessary for odor control at the sites. The diary farmers were slowly being squeezed out of business due to economic conditions and no other nearby sites were available to allow incorporation of biosolids. The Superintendent developed a windrow composting program utilizing the biosolids storage shed for part of the year in 1987. About half of all the biosolids produced were composted and half land applied through 1988. All biosolids produced were composted when the last dairy farm closed in 1989.

RBC Shaft weight became excessive, threatening premature shaft failure whenever wastewater temperatures dropped below 10 degrees Celsius. This condition generally occurred from November through May and all attempts to control shaft weights were unsuccessful. Numerous control measures were tried including air purging, in-line chlorination, isolation and starvation, and chemical stripping but all were unsatisfactory because they either did not remove the excess biomass or when they did, effluent quality suffered.

The Superintendent found the solution to the RBC shaft weight problem while attending the January 1994 NEWEA Conference when a paper presentation by Kenneth E. Neu discussed upgrading RBC systems to achieve higher quality effluent thru solids recirculation. If the treatment plant could recirculate secondary solids (and in effect run an activated sludge system in concert with the RBC’s) it would reduce shaft weights and provide better treatment. Unfortunately there was no primary clarification at that time so the idea was put on hold.

Despite the best efforts of plant operators, by 1997 the District was faced with a continuing CSO problem and it became clear that an upgrade to the plant was needed to eliminate the CSOs. An opportunity presented itself to eliminate several problems with one upgrade. In 1999 the upgrade was constructed to include primary clarification and secondary solids recirculation. The district direct purchased and installed two return pumps and portable blowers for conversion of the windrow composting to aerated static pile. The upgrade eliminated composting odor problems, eliminated the CSO and eliminated the excessive shaft weight problem.

The ability to re-circulate secondary solids improved treatment and reduced power consumption. The average percent removal for the period July thru October 2000 for BOD was 96.6% compared to 89.9 % for the same period in 1999. TSS average percent removal was 95% compared to 89% for the same period in 1999. Energy consumption has dropped because air required to turn the RBC’s is less when biomass load on the shaft is less and because the need for a 15 horsepower mixer in the blended sludge tank was eliminated. Secondary solids are now co-thickened in the primary tanks.

The Lincoln facility was the first and only RBC plant in the state of Maine to re-circulate secondary solids.