Like many municipal wastewater districts, we use lagoons for treatment and disposal. And like lagoons everywhere, ours require dredging.
Our Southeast Regional Wastewater System in Clearlake, Calif., has two lagoons, each treating about 0.5 mdg. The effluent is commingled, chlorinated, and sampled weekly and then reported as a monthly average. The effluent leaves each pond from a corner surface-discharge pipe.
We had spent three weeks in November 2013 mechanically dredging the South Lagoon, removing 478 dry tons of sludge. In December, we estimated the North Lagoon had about 378 dry tons in it.
This forced us to all but shut down the pond due to carryover of the alluvial sludge, which goes directly into our chlorine contact ditch. The ditch slowly fills in, causing short-circuiting and less contact time, which makes it difficult to comply with California’s discharge requirements.
We were paying a dredging contractor $1,000 per dry ton, which translated to about $150,000 annually. That didn’t include inspecting and cleaning the contact ditch at an additional $15,000 to $20,000 per year.
We needed a more cost-effective alternative to mechanical dredging.
Making nature work faster
We also wanted to be good environmental stewards. Our dual goal was to identify affordable treatment and disposal methods that have a positive impact on the receiving environment.
In spring 2014, after extensive research and analysis of available alternatives, we entered into a paid pilot study with Absolute Aeration LLC of Greeley, Colo., of the company’s hybrid aeration technology.
The chemical-free system targets sludge-digesting bacteria in the lagoon that form synergistic anaerobic biofilms in tight, mineral-based granules. These biofilms spread out to create a granular sludge bed reactor (GSBR) over the entire bottom. Surface biosolids are delivered to the GSBR, liquefied, and turned into gas by the bacteria immobilized in and on the granule. The gas rises and gently mixes the water column, continuously feeding the granules to increase their productivity.
The process was discovered after years of testing in agricultural wastewater ponds by the system’s inventors.
Our pilot study involved three of the company’s products:
Blue Frogs. Base circulator that acts as a mixer and passive aerator to create radial outflowing currents that increase biological activity with minimal energy consumption. Moves up to 7 mdg.
We were attracted to this solution because it would eliminate dredging as well as our high-horsepower aerators.
Sludge levels cut in half in three months
The project began with three weeks of preparation by our Southeast Treatment Plant crew, followed by four days installing seven Blue Frogs, 10 Yellow Frogs, and one Gold Frog with Absolute Aeration’s team.
Using the weighted disc method to find starting sludge depth, 14 sites across the North Lagoon were marked and mapped for repeatability.
At 2:30 p.m. on Oct. 30, 2014, we put the system online. To our delight, we began experiencing results almost immediately.
Eighty-six days later, the system had eliminated 49% of the stored sludge, reducing the sludge depth from 5 feet to 2.55 feet. By the second quarter, it had dropped by almost 62% (see chart). As of March 2016, 18 months later, the lagoon was essentially sludge-free.
We’re fine-tuning the process to support our compliance requirements and preparing to install the system in the South Lagoon later this year.
Also, the plant’s power bill fell 16% to 18% for the year, yielding an estimated annual savings of $34,000 to $36,000. Once we complete the South Lagoon treatment process, we project total annual savings of $60,000 to $62,000. That will cover the cost of two new operator trucks or one utility maintenance truck, as well as a new lateral inspection camera with a recording head.
Future savings from power reductions and dredging will be reinvested in our aging infrastructure.
John Sparkes is site superintendent for the Southeast Regional Wastewater Collection and Treatment System in Clear Lake, Calif. E-mail firstname.lastname@example.org.