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The following is taken from the Executive Summary of the Preliminary Engineering Report, November 2014 prepared by HDR Engineering, Inc.:
Howard County is proceeding with a Biosolids Processing Facilities Improvements project (the Project) at the Little Patuxent Water Reclamation Plant (LPWRP). The Project will implement the recommendation from the County’s 2013 Biosolids Master Plan Study to replace the current biosolids stabilization practice, consisting of advanced lime stabilization of undigested primary and waste activated solids, with anaerobic digestion and direct heat drying. While both stabilization processes produce Class A, exceptional-quality (EQ) biosolids, beneficial use of lime stabilized biosolids is limited to bulk agricultural land application, which is becoming more restricted under Maryland regulations. Anaerobic digestion and heat drying produces a more versatile biosolids product suitable for a variety of beneficial uses. The recommended improvements provide the County with reliable, cost-effective, and socially responsible treatment and beneficial use of LPWRP biosolids in a changing and unpredictable regulatory environment, as elucidated in the objectives statement for the Master Plan Study.
The recommended improvements also provide to the County the benefits of biosolids volume reduction and annual operations and maintenance cost savings. As shown by the comparison in Table ES-1, replacing advanced lime stabilization with anaerobic digestion and heat drying is expected to reduce biosolids volume and truck traffic by over 80 percent and save almost $2 million/year in annual operations and maintenance costs.
Table ES-1 Biosolids Improvements Volume Reduction and Cost Savings
Anaerobic Digestion and Heat Drying
Type of Class A, EQ biosolids produced
Limed Dewatered Cake
35% to 40% solids
90% to 95% solids
Total biosolids volume, wet tons/year
Biosolids hauling, average trucks/day
Biosolids operations and maintenance, $/year
The Preliminary Engineering phase of the Project was undertaken to:
· identify viable beneficial use markets for dried biosolids in Maryland and neighboring states;
· determine which drum and belt dryer options can reliably produce the type of dried biosolids preferred in each viable beneficial use market;
· evaluate and select the recommended alternatives for drying, anaerobic digestion, and supporting solids handling process improvements;
· refine design criteria, equipment configurations, and facility layouts for the recommended solids and biosolids improvements to serve as the basis for final detailed design; and prepare a preliminary opinion of probable construction cost for the Project for the County’s capital improvements program budgeting.
Key decisions that emerged from the Preliminary Engineering evaluations to align selected alternatives and preliminary design configurations with the County’s biosolids management objectives are summarized in Table ES-2.
Table ES-2 Key Preliminary Design Decisions
Soil blending is the primary target market for LPWRP dried biosolids; agricultural land application is the secondary market
Market survey showed a viable Maryland soil blending market with significant interest in and capacity for dried biosolids, but little interest for dried biosolids in local specialty fertilizer markets
Provide belt dryers for the LPWRP; final selection of the specific dryer model to be made during final design
Belt dryers produce a granular dried biosolids product that is suitable for agricultural uses and preferred by soil blenders over the spherical pellets produced by drum dryers.
Repurpose two anaerobic pretreatment reactors as anaerobic digesters and add a third digester of equal size
Provides operational flexibility; retention times for well- stabilized, low odor solids to drying; and reliable Class B biosolids as a backup to drying.
Minimize encroachment of new facilities on lime stabilization and staging area
Maintain and minimize impacts to lime stabilization, staging and hauling operations during construction.
Co-locate centrifuges and belt dryers
More efficient dewatering and drying operations and avoids pumping/conveying dewatered solids long distances
Provide sidestream centrate treatment
Maintains nutrient removal capacity in mainstream process for future growth
Provide new gravity belt thickeners for waste activated solids (WAS) thickening
More consistent and thicker solids feed to anaerobic digestion improves process performance
Major elements of the recommended improvements for the Project are shown on Figure ES-1 and summarized in Table ES-3.
Table ES-3 Summary of the Recommended Improvements
Process or Facility
New Dewatering and Drying Building
Two direct belt dryer systems
Odor control biofilter for dryer exhaust
Dried biosolids storage silos and truck loading station
Backup dewatered solids truck loading station
Relocate three existing centrifuges to new Dewatering and Drying
Convert two 1.75 MG anaerobic pretreatment reactors into anaerobic digesters with gas holder membrane covers and pumped mixing
Add third 1.75 MG anaerobic digester with fixed cover and pumped mixing New Digester and Boiler Buildings housing digester pumping, mixing and heating equipment
Convert existing aerobic pretreatment basins into centrate storage and a deammonification treatment process for nitrogen removal
Digester Gas Handling
Gas treatment system, including hydrogen sulfide removal, moisture removal and compression, to allow use of digester gas as dryer fuel New enclosed digester gas flare
Table ES-3 Summary of the Recommended Improvements (continued)
Repurpose Thickened Primary Solids Storage to Digester Feed Tanks Convert Thickened WAS Storage to Digested Solids Storage with mechanical mixing