Imaginative approach turns wastewater challenge into an opportunity

When design was underway to upgrade Pennsylvania’s Milton Regional Sewer Authority (MRSA) WWTP, demand for the neighbouring ConAgra site’s food products had led to an increase in factory production, creating an organic overload in the treatment plant’s aeration tanks.

Due to the anticipated removal of ConAgra’s settling processes, the strength of the food producers’ wastewater was expected to rise by up to one third.

Seeking a cost-effective wastewater treatment solution that would also resolve high sludge disposal charges and mounting energy costs, plant operators at Milton saw the opportunity for anaerobic pre-treatment. This would not only reduce the load on the aeration tanks, but also provide renewable energy in the form of biogas.

They turned to Landia which installed two full-scale 7.5 million-gallon (28,000 m³) reactors were installed – designed to remove approximately 90% of the organic load from ConAgra’s wastewater, which totals between 1 and 1.5 million gallons per day. As well as the wastewater, the reactors can also receive septage and other hauled wastes.

In addition to the new 2.4 MGD Anaerobic Treatment Process, Milton WWTP now includes a 4.25 MGD Biological Nutrient Removal system – and it was here, as Victor Derr, Superintendent at Milton Regional Sewer Authority, explains, that the large, wide channels of the new vertical loop reactor were causing problems.

“The flow was very low”, he said, “so everything kept settling out. With ConAgra’s wastewater and the 7,000 people in the Milton area that the plant serves, our loading was off the chart. No nutrients and very high BOD. We had to keep solids in suspension but two of our channels couldn’t be taken out of service”.

A solution was found by installing four submersible mixers made by Landia. These Landia model POP-I gear-driven mixers, complete with guide rail assemblies and service platforms, were installed in the influent, effluent and return channels of the nutrient removal tank system.

The mixers, two 2.4HP and two 4.9HP units, were specifically sized for each of the channels to provide optimum mixing in the areas where the plant was previously experiencing an accumulation of solids.

To avoid conflict with an existing 20-inch sub-surface pipe, which runs through the return channel and the influent channel, Landia supplied custom-built service platforms to allow for mixer installation without draining the tanks. Landia’s customized solution allowed the plant to continue operating with no interruptions throughout the mixer installation phase.

The upgrade at Milton expanded wastewater capacity from 3.42 MGD to 4.25 MGD and also doubled its organic capacity to nearly 50,000 pounds per day.

The dewatering process of removing sludge from the reactors in to a holding tank created another issue.

Said Derr: “The belt would appear to be running fine, then there’d be a washout with water, followed by a very heavy pocket of sludge. This meant we couldn’t really leave it unattended because sooner or later we knew it was going to stratify”.

A fifth submersible mixer from Landia was installed in the sludge decant tank to homogenize the sludge prior to dewatering.

“It now takes care of itself, as it should.  Before we had the Landia mixer, we were running at 50 gal/min – and having to put in lots of time, but now, without the previous vortex, we’re up at 85 gal/min, with no sign of those washouts or the big pockets of sludge”. 

As a by-product, the reactors create valuable biogas containing approximately 75% methane, which is burned in two 1,000 kW gas engines to generate electricity -  sufficient to offset the power the wastewater treatment process utilizes. 

Low-rate reactors produce approximately 60% less sludge than aerobic wastewater treatment lowering sludge disposal costs and saving landfill.