Biosolids is a term applied to treated sludge that meets requirements for beneficial reuse through application to agricultural land and use by the public. The treatment and distribution of biosolids is governed by federal regulations contained in 40 CFR 503. These regulations set criteria and process requirements pertaining to various classifications of biosolids. The District has prepared and submitted to the State Division of Water Quality a written Biosolids Management Plan. The Biosolids Management Plan sets forth the District’s methods for complying with the 503 regulations and declares the specific processes for achieving compliance. As stated in the plan, “The goal of the District is to maintain compliance with all requirements to achieve ‘exceptional quality’ biosolids.” “Exceptional quality” is an EPA term to define biosolids that meet the highest level of quality in three categories: 1. Reduction of pathogens; 2. Reduction of vector attraction (odor); and 3. Low concentrations of specific metals. The biosolids produced by the District meet the highest level in each of these categories and are designated “exceptional quality.” As such, the biosolids given away by the District have relatively few restrictions for use. They can, in fact, be used in home gardens. 

 The treatment and preparation of biosolids begins during the solids treatment process and  is completed on a large concrete slab storage area. As solids are processed in the digesters they are decomposed and stabilized. It is through the digestion process that the volatile solids, which contribute to odors, are reduced and pathogens begin to be reduced. The regulations require a minimum 38 percent reduction in volatile solids. The average reduction achieved by the District’s digestion process is 45 to 60 percent. 

After treatment in the digesters, solids are pumped to the solids dewatering facility.  Solids pumped from the digesters to holding tanks at the solids dewatering facility are at an average solids concentration of 1.5 to 3.5 percent.  The purpose of the dewatering facility is to increase the solids concentration, or dryness, of the solids to between 18 and 22 percent.  At these higher concentrations the solids will stack and can be turned and mixed.

The District operates belt filter presses to dewater and concentrate the solids content.  Belt filter presses make use of two mesh belts which allow water to pass through the belt while retaining the solids on the belt.  After an initial gravity separation of liquid and solids, the two belts come together and sandwich the solids.  The belts then pass in a serpentine pattern through a set of rollers which create pressure on the solids and squeeze additional water out.  Concentrated solids discharge from the belt presses is called sludge cake.  The cake is conveyed from the belt presses and is discharged to a truck and trailer rig.  When the trailer is filled, the sludge is driven out and discharged onto a concrete processing/storage pad.

On the storage pad the biosolids are stacked in rows about 7 to 18 feet high and about 20 feet wide. The processes of decomposition, stabilization, and disinfection continue in these piles.

As the solids are in the piles on the storage pad, moisture content and temperature are monitored and the piles are mixed and turned periodically. The moisture and turning are important in promoting biological activity in the piles. Turning the piles introduces air. The combination of air, moisture, and bacteria with the organic matter in the biosolid creates the conditions necessary for decomposition. This process is very similar to garden composting. Heat is generated through the natural biological activity in the piles. Temperatures of up to 140˚F can be achieved in the piles. These high temperatures further reduce pathogens and disinfect the biosolids.

After the biosolids have been treated and stored in piles for 8 to 12 months, they are sampled and analyzed for pathogenic and metals content to determine compliance with the regulations. The required pathogen tests include salmonella, fecal coliform, helminth ova, and enteric viruses. The metals analyzed for are arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. Once the laboratory analysis reports are received and compliance with Class A pathogen reduction requirements and Table 3 metals limitations as specified in the 503 regulations, the biosolids become available for distribution to the public. The District produces approximately 2,500 dry tons of biosolids each year. The District has been successful at disposing of the biosolids through the give-away program. There are many repeat customers who have experienced the fertilizing and soil enrichment benefits of biosolids and use it enthusiastically.