March 17, 2011
FLU$HED AWAY -Proposed DNR Lagoon Permit Could Require Costly Upgrades For City
Proposed changes to the Missouri Department of Natural Resources (DNR) permit for the wastewater treatment system operated by the City of Memphis may mean significant chances for the municipal sewer provider.
Engineer Cary Sayre of Allstate Consultants, LLC, met with the Memphis City Council on March 3rd to discuss the pending permit for the city lagoon.
The permit, which is renewable every five years, has been in the works since the spring of 2010. The new guidelines, if granted final DNR approval, will mean significant changes for the city.
The most significant modification to the permit is DNR's insertion of limits on Ammonia generated in the lagoon system. Previously the permit had simply required regular testing for the possible pollutant. Under the proposed permit, the city lagoon would soon be limited to Ammonia production of less than 3.1 mg/L in summer months (May 1-October 31) and 3.7 mg/L in winter months (November 1 - April 30).
Superintendent Dennis Howard reported those numbers will be nearly impossible to meet under the current lagoon conditions.
Sayre agreed, adding the only way to reduce ammonia levels to the allowable levels likely would be the construction of an aeration system for the lagoon. In addition to the high cost of such an upgrade, Sayre noted that aeration would also produce additional sludge in the lagoon, which possibly could trigger additional testing problems for that portion of the permit.
Under the proposed permit, the city would have three years to come into compliance with the new ammonia limits.
The council members discussed with Sayre the practicality of making costly upgrades to the lagoon and the likelihood of additional permit changes in the future that would only signal more costly upgrades.
The engineer indicated he is currently working with at least one other municipality in a similar situation, and that community is studying the feasibility of the construction of a fully-contained waste water treatment facility. Sayre estimated the cost at $2.6 million for the facility. Based on the smaller service area, rough estimates were offered in the $3.5 million to $4 million range for such a facility to be built to serve Memphis.
The sewer system will be on a shorter chain with DNR on addressing hydraulic overloading of the lagoon.
The previous permit had identified concerns regarding the lagoon exceeding designed flow rates. The structure, which was originally built in 1969, was designed to serve a population of 2,143 with a design flow of 214,300 gallons per day. Actual flow at the lagoon according to DNR records is averaging 325,260 gallons.
That number is actually down from recorded levels under the previous permit, but the DNR is ratcheting down its efforts to bring the lagoon system into full compliance.
Under the new permit, the city will be required to conduct an engineering study to address the hydraulic overloading of the Memphis Municipal Wastewater Treatment Facility. The study will reveal if upgrades or improvements to the facility will be required.
The last major modifications to the site occurred in 1985 when baffle curtain dividers were installed to divide the lagoon into a three cell system to aid in sludge retention. The curtains lengthen the time wastewater is retained in the lagoon by creating a three-phase flow from the sewer inflow to where the treated water actually exits the system into Gunns Branch. However the upgrade also lowered the design flow of the system as it divided primary cell, reducing its size.
The municipal wastewater system was in full compliance according to the last DNR inspection on July 6, 2009. The inspector gave the receiving stream a clean bill of health, but did not that the retaining walls were beginning to show signs of age.
A facility inspection in July 2010 by the US Environmental Protection Agency (EPA) resulted in a issue of warning after the effluent flow meter was determined to be out of calibration and was underestimating the amount of water flowing out of the lagoon system. The inspection report noted no concerns with the environmental quality of the receiving stream area, but did note concerns about the lagoon being hydraulically overloaded (too full of water).
The EPA report also noted that the facility had met all permitted limits during the past permit cycle.