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The Giant Step From Lagoons to Mechanical Treatment Print E-mail
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Tuesday, 25 March 2008

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Some Michigan communities are facing the difficult task of upgrading or expanding their facultative lagoons that discharge twice a year. The upgrading or expansion is like a two-edged sword with respect to cost. The capital costs as well as the operation & maintenance costs can potentially take giant leaps into higher ground unless the project is given the closest of scrutiny.

The capital costs are high because lagoons now need to be sealed with a plastic liner placed over an impermeable clay base. Previously, either plastic or clay was required. Now both are needed. This makes the capital cost of a mechanical treatment plant competitive with the low-tech lagoons. The answer seems simple. Just build a mechanical treatment plant.

Whoa!! The operating and maintenance (O & M) costs of a mechanical treatment plant are about five times that of a lagoon. The result is not palatable. The capital costs may raise the water and sewer rates by 30% and the O & M costs may raise the rates by another 30% for a total of a whopping 60%.



In many cases, both the lagoons and the treatment efficiency can be upgraded while maintaining the status of semi-annual discharge. This would involve the construction of a low maintenance treatment plant that pretreats the sewage before it enters the lagoons. During the pretreatment, the organic loading to the lagoons would be reduced by 60%.

How will the lagoons operate under the lower loading? There is local experience for this situation. In two facultative lagoons serving Michigan schools, an unexpectedly high effluent quality was achieved. Upon analysis of water use records, it was determined that the wastewater flows from the schools are less than half of the flows that occurred several years ago. This agrees with the experience inside the school where the shower and toilet facilities are used far less frequently these days. As a result, the normal organic and hydraulic loadings were likewise reduced.

There is a biological treatment plant that includes only two motors that would be appropriate for this low maintenance application. In addition, the treatment plant operation is less critical because it does not continuously discharge to the waters of the State.

With this approach, the costs of the expansion or upgrading can be reduced by a third.
Last Updated ( Thursday, 03 July 2008 )
 
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