The Challenge

The City’s previous method for biosolids handling (filter press) resulted in a dewatered sludge, which produced truckloads of biosolids that had to be shipped out every year to local farms or to a landfill. It was costing the City a significant amount of money to regularly transport this large volume of dewatered sludge.

Our Solution

Process alternatives in our study included belt filter press and centrifuge dewatering equipment, as well as thermal dryers, solar drying, lime pasteurization for Class A biosolids, and the disposal of Class B biosolids. Our optimal solution would be less costly and efficiently remove sludge from the waste stream at its WWTP. The study pointed the City towards choosing to install a thermal dryer system to produce class A biosolids.

The project design included:

• new sludge pumps
• a belt filter press
• dewatered solids storage
• screw conveyors
• thermal belt dryer system
• a bucket elevator
• building improvements
• biosolids storage area improvements
• site work
• piping
• valves
• electrical
• controls
• supervisory control and data acquisition (SCADA) programming
• related appurtenances

The Results

The City selected the thermal belt dryer system as the Class A biosolids system. The installation of a thermal dryer not only reduced the volume of biosolids waste, but reduced pathogens and created a Class A compost product that can be used in many applications.

Biosolids leave the aeration tanks as 90% wet, then are reduced to 20% wet after going through the belt filter press, before getting down to only 1% wet when the biosolids come out of the thermal dryer (which significantly decreases the volume of the sludge by removing the water). This is the primary benefit of a thermal dryer, but a secondary benefit is that it kills the pathogens.

This new process alters a waste product from the aeration process into a beneficial reuse as a soil amendment that provides several benefits, including nutrient additions, improved soil structure, and water reuse. Biosolids are rich in nitrogen, phosphorus, potassium, sulfur, magnesium, calcium, copper, and zinc, which are all beneficial elements for plants. Class A biosolids compost produced by the Lenoir WWTP can be bagged and sold as a soil amendment that it is safe to apply to gardens or be used by NCDOT for roadside soil backfill. Utilizing a thermal drying process results in a product that can be sold or used, which is a benefit to the City.

At its permitted capacity, the City would utilize its belt filter press and thermal dryer to reduce approximately 98,000 wet tons of sludge to 1,600 dry tons of compost annually. At current capacity in 2023, the City dewatered 154 million gallons of sludge with an average of 2.05% solids, equivalent to 767 dry tons per year.

Through the implementation of this new system, the City of Lenoir increased annual expenditures for the purchase of natural gas used in the drying process (estimated to be $84,000 per year), but also reduced expenditures associated with hauling the biosolids to local farms (estimated to be $73,000 per year) and operations and maintenance costs (estimated to be $8,000).

After our team completed the biosolids study, the City then retained McGill for the design, permitting, bidding, and construction administration of biosolids facility improvements at its Lower Creek WWTP.