To Digest or Not to Digest -- An Updated Evaluation of an Age-old Question of Carbon Management in Water Resource Recovery Facilities

It has been accepted wisdom in the wastewater industry for many years that primary treatment combined with anaerobic digestion becomes more cost-effective the larger the size of the water reclamation facility, whereas at smaller facilities waste activated sludge (WAS) only systems are more commonly used. Primary treatment reduces the organic load on secondary processes, leading to aeration savings, and anaerobic digestion provides opportunities for beneficial use of biogas. However, for facilities with nutrient removal requirements, diverting carbon to anaerobic digestion can lead to a deficit of carbon for nutrient removal needs, and lead to costly addition of supplemental carbon. Meanwhile, treatment technology is constantly improving and changes in technology and green initiatives can have a significant impact on the optimum solution for carbon diversion. Several considerations are outlined below. ...Enhanced primary treatment (e.g. primary filtration): leads to higher carbon diversion to digestion and less carbon to secondary processes, reducing aeration basin volume and aeration costs, but potentially leading to higher supplemental carbon needs if nutrient removal is required. ...Primary sludge fermentation: can be used to solubilize some of the carbon in primary sludge for diversion back to liquid stream processes to meet nutrient removal needs, while also minimizing aeration basin volume that would otherwise be driven by additional aeration basin influent solids without primary treatment. Sidestream Enhanced Biological Phosphorus Removal (S2EBPR): provides the opportunity to ferment return activated sludge (RAS) or mixed liquor to drive biological phosphorus removal via carbon in the RAS/mixed liquor, reducing reliance on influent carbon or supplemental carbon. ...Partial denitrification anammox (PdNA): is an emerging process that has been proven at full scale and which has significant potential to reduce carbon needs for denitrification and also reduces aeration demand. Both S2EBPR and PdNA offer the potential to reduce supplemental carbon needs for nutrient removal, freeing up more carbon for diversion to anaerobic digesters. ...Renewable Identification Numbers (RINs): are green trading credits applied to the production of renewable fuel that incentivize the production of renewable natural gas (RNG) from biogas at wastewater facilities with anaerobic digesters. In addition to the current RIN system that incentivizes RNG, a pathway for electrical RINs (eRINs) may soon be available that could incentivize production of electricity (e.g. via combined heat and power systems) This paper will provide an economic analysis evaluating the impact of various factors on the decision of whether or not to implement primary treatment and anaerobic digestion at water reclamation facilities. Factors to be evaluated will include the following: 1.Facility size 2.Efficiency of primary treatment carbon capture (conventional versus enhanced primary treatment) 3.Nitrification only facilities compared to those with nutrient removal requirements for nitrogen and phosphorus 4.Implementation of primary sludge fermentation to provide more carbon to meet nutrient removal needs 5.Implementation of S2EBPR for more efficient use of carbon for P removal 6.Implementation of PdNA to reduce carbon needs for total nitrogen removal 7.Cost sensitivity to green incentives (e.g. RINs) for biogas upgrading To provide an example of the type of analysis that will be included in the paper, a preliminary life cycle cost analysis has been conducted for various sizes of facilities for the simplest scenario where only nitrification is required (no nutrient removal requirements). The full paper will expand on this analysis to develop life cycle costs to evaluate the factors outlined above. The following approach was taken: 1.Process unit sizing was developed for a range of facility sizes from 5MGD to 100MGD 2.Two scenarios were evaluated for each facility size, one with primary clarification and anaerobic digestion and the other with no primary clarification. 3.Capital, operating and life cycle costs were developed for each size of facility for each scenario. Details of the scope of capital costs included for each scenario are provided in Table 1. In general, the scope of capital and operating cost development was limited to items that would differ between the two scenarios being investigated. Details of the scope of operating costs included and key assumptions are provided in Table 2. It is acknowledged that the assumptions made inevitably impact the outcome of the evaluation and sensitivity to a range of assumptions will be included in the final paper. Net present costs were developed using a 2% inflation-adjusted rate over 20 years. Results are shown in Figure 3 and Figure 4. Key take-aways from this preliminary analysis are as follows: 1.Capital cost for the scenarios utilizing primary clarification and digestion were higher than those with WAS only activated sludge 2.Operating costs for the WAS only activated sludge scenarios were higher than those with primary clarification and digestion 3.As expected, economies of scale led to primary treatment and anaerobic digestion becoming more cost-effective as facility size increases The full paper will elaborate extensively on this analysis, providing utilities with useful insights to help understand under what conditions primary treatment and anaerobic digestion is favorable compared to WAS only activated sludge.