Which Class A Drying Process Has the Most Advantages for the Dry Creek Water Reclamation Facility, Cheyenne, Wyoming

Introduction The Dry Creek Water Reclamation Facility (DCWRF), owned by the Cheyenne Board of Public Utilities (BOARD), is located in Cheyenne, Wyoming. Thickened waste activated sludge and primary sludge are anaerobically digested and the digested biosolids are pumped to high solids centrifuges for dewatering. The biosolids are transported across the site to air drying-beds for storage and supplemental air drying prior to offsite transport for beneficial use land application. A site map of the existing DCWRF biosolids processing sites are shown in Figure 1. DCWRF generates approximately 1,600 metric tons per year of primarily Class B biosolids. Some Class A biosolids product is produced by complying with the 'two-summer method' as allowed by the United States Environmental Protection Agency (USEPA). The air-dried Class A and Class B biosolids are land applied by the DCWRF staff at private ranches. Much of the soil in the region has low organic content and it is a challenge to successfully grow native grasses and crops without a significant amount of fertilizer. Therefore, the ranches benefit from the application of the organically-rich biosolids material. Challenges with the Existing Solids Processing Facilities The utility is currently facing numerous solids processing challenges including aging equipment as well as a potential loss of nearby ranch sites that had been used for land application. The uncertainty in the continued acceptance of Class B biosolids, due to change in ownership of the ranch sites currently accepting biosolids, will result in lower reliability and flexibility for solids beneficial use for the plant in the future. Faced with this potential future challenge, the BOARD selected Jacobs Engineering Group to evaluate Class A stabilization technologies that are appropriate for the needs of the utility. The production of Class A biosolids material that has multiple end-use markets would give the BOARD more reliability and flexibility to land apply in locations such as the City of Cheyenne's parks, open spaces, golf courses and sporting fields which would reduce hauling cost and the reliance on private farmers and ranchers. Ranchers would also be more receptive to land application of Class A biosolids. Objectives of the Evaluation The objective of this study was to compare the suitability and applicability of three different Class A drying alternatives including belt drying, solar greenhouse drying and hybrid drying (solar greenhouse and air drying) to DCWRF. The three alternatives were evaluated based on capital cost, operations and maintenance (O&M) cost as well as life cycle cost considerations. Additionally, a combined financial and non-monetary ranking and sensitivity analysis was performed. The advantages and disadvantages of each drying alternative, considering the plant's geographical constraints, were outlined. Alternatives Evaluation The thermal drying and solar drying equipment preliminary design criteria is shown in Table 1. The three alternatives evaluated are summarized below and a screw press was assumed to be the selected dewatering technology. The process schematic for Alternatives 1, 2 and 3 are shown in Figures 2, 3 and 4. The advantages and disadvantages of each alternative will be described in detail in the paper and presentation. The proposed solar greenhouse configuration location on the existing air-drying beds is shown in Figure 5. The proposed configuration of the hybrid solar greenhouse structure solar greenhouse located adjacent to the existing air-drying pads is shown in Figure 6. Results of the Evaluation Non-Monetary Evaluation Several non-monetary evaluation criteria were used in the assessment of the three drying alternatives. The results of the weighted benefit scores based on the DCWRF staff input are summarized in Table 2. Based on non-monetary considerations, belt drying and solar drying were shown to be the most beneficial to DCWRF, with essentially the same score, with solar drying being slightly better, while the hybrid alternative represented the least non-monetary benefits. The belt dryer ranked significantly higher in the product end use flexibility criterion. The hybrid and solar drying alternatives had lower scores for the odor generation criterion because operators are exposed to these odors for extended periods of time compared to belt drying odor exposure. Based on non-monetary considerations only, solar drying is the best alternative for the DCWRF, but belt drying is ranked approximately the same. Life Cycle Cost Evaluation In addition to non-monetary considerations, opinions of probable construction cost, annual O&M cost and net present worth costs were estimated for the three drying alternatives. Life cycle costs comparison are summarized in Table 3. The solar drying alternative had the highest life cycle cost followed by the belt drying alternative; while the hybrid drying alternative had the lowest life cycle cost. Based on relative cost ranking, the solar drying option is approximately 40 percent higher than the other two options. Combined Ranking and Sensitivity Analysis The cost rankings and non-monetary rankings for the different alternatives were combined and the total weighting rank was used to determine the best overall drying alterative. The combined ranking included the following three monetary to non-monetary ratios: 1. 30 percent weighting for cost and 70 percent weighting for non-monetary criteria 2. 50 percent weighting for cost and 50 percent weighting for non-monetary criteria 3. 70 percent weighting for cost and 30 percent weighting for non-monetary criteria The belt drying option ranked highest against all three weighing ratios and is therefore the preferred option. Conclusion The belt drying option ranked highest on all three weighting ratios, had the highest non-monetary ranking and the most advantages and was therefore the preferred option for the DCWRF. This presentation will be valuable to owners, operators and consultants that want to consider Class A drying alternatives. In addition to presenting the results of the study, advantages and disadvantages, preliminary and final design criteria, equipment layouts and cost estimates will be presented for the three drying alternatives.