Maintenance cleaning has begun on lagoons at two of NLRW’s water reclamation facilities.
Upkeeping wastewater lagoons is vital to the operations of water reclamation facilities (WRFs). This year, NLRW is cleaning out lagoons at Faulkner Lake WRF and Five Mile Creek WRF.
Faulkner Lake WRF is a mechanical plant, meaning wastewater treatment takes place in a series of mechanically operated concrete basins with pipes that transport the wastewater from basin to basin. The lagoons serve as storage for biosolids, the pieces of solid material removed from wastewater. Five Mile Creek WRF is a lagoon plant, meaning that lagoons facilitate the primary treatment process, providing a place both for microorganisms to break down organic matter and for excess material to settle out of the wastewater.
Despite the differences in purpose, the lagoons at these plants share some basic traits, including slowly filling with biosolids over time. The biosolid level becomes problematic if it interferes with aerators or leaves insufficient space for water.
Biosolids Removal
NLRW has an annually renewable contract with DRT Biosolids Inc. for lagoon cleaning. This year, DRT Biosolids Inc. will clean out two lagoons for NLRW, one at Faulkner Lake WRF and one at Five Mile Creek WRF.
Faulkner Lake WRF
Faulkner Lake WRF has two lagoons, both of which were taken out of service in 2016 when the high-water lift station was built. They remained out of service until 2021, when they were reopened to store biosolids.
This year, the east lagoon remains in use while the west lagoon is getting cleaned out. A sludge crawler traverses the west lagoon throughout the day, resuspending the biosolids in the water so the mixture can be pumped into semitrucks. Approximately fifty truckloads are hauled off the premises on a typical day.
The biosolids removal process is complete when cleaning efforts start removing the lagoon’s clay liner. This is determined by measuring the prevalence of volatile solids, which reflects the prevalence of organic matter. Bacteria break down organic matter over time, resulting in decreasing prevalence the deeper one tests in the lagoon. Typically, the biosolids near the top – and thus removed early in the cleaning process – contain around 30% volatile solids. When samples begin returning about 20% volatile solids, cleaning efforts are stopped as they have begun removing the clay liner. Then, the west lagoon can be returned to service and the east lagoon can begin dewatering in preparation for its next cleaning.
Five Mile Creek WRF
In contrast to the lagoons at Faulkner Lake WRF, the lagoon at Five Mile Creek WRF must remain in service throughout the biosolids removal process. This will require the use of a dredge. A dredge is a specialized boat equipped with a mixer and pump. The boat traverses the water’s surface while extending the mixer and pump down to the bottom of the lagoon. The mixer and pump work as a vacuum, pumping the sludge from the lagoon into a frac tank for storage until it can be pumped into semitrucks and hauled off.
Biosolids Reuse
There are currently four practical ways to dispose of biosolids: incineration, landfilling, pelletization, and land application. Of those four, NLRW’s current infrastructure allows for two: landfilling and land application. This leaves land application as the most environmentally friendly and cost-effective option available.
The amount of biosolids that may be applied to a plot of land is determined primarily by nitrogen content. The maximum weight of nitrogen that can be applied to the land is determined based on the type of crop and the amount of nitrogen already in the soil. The nitrogen content of the biosolids is measured before being spread on the land. When a piece of land reaches its nitrogen capacity, no more biosolids are applied that calendar year.
Another concern is the soil’s pH. In order for natural processes to safely and effectively filter the biosolids, the soil must have a pH of 5.7 or greater. If the pH is below the threshold, it is increased by applying agricultural lime, which has the additional benefit of increasing plants’ root growth and nutrient absorption, before applying any biosolids. Estimates predict needing 900 tons of lime this year.
Precipitation is the largest determinant of when biosolids are applied to a piece of land. Rain causes surface runoff, but the goal with land application is for the biosolids to stay where they are placed. Therefore, lagoon cleaning operations shut down when the 24-hour forecast includes a 50% or greater chance of rain. This is the primary cause of uncertainty in the project timeline: with no rain, the cleaning process would take a handful of weeks to complete, with lots of rain, it could take months.
An NLRW representative is present at the application site at all times during land application to ensure that all environmental requirements, policies, and best practices are followed.
Future Lagoon Cleaning
Lagoons are cleaned out on a schedule determined by need. The lagoon at Five Mile Creek WRF has steadily required cleaning annually. The equalization basin at Maumelle WRF was last cleaned in 2017, and the lagoon at White Oak WRF was last cleaned in 2018. Based on the estimated build-up rate of 16 to 18 hundred dry tons of biosolids per year, Faulkner Lake WRF is predicted to need a lagoon cleaning every 1 to 2 years.
