Full Scale Pilot of a Novel Struvite Precipitation System at the Provo WRP

INTRODUCTION The Provo Water Reclamation Plant (WRP) which treats approximately 15 MGD of influent is currently undertaking a major plant upgrade to address effluent phosphorus levels. This upgrade includes a mainstream liquid process shift to enhanced biological phosphorus (P) removal (EBPR). This upgrade also has implications for the solids stream as historically the plant has struggled with nuisance struvite formation in its dewatering centrate downstream of its anaerobic digesters. The introduction of EBPR is expected to exacerbate the struvite issues by increasing the ortho-P concentrations in digested sludge from the currently levels of 60-70 mg/L to levels approaching 200-250 mg/L. This struvite issue combined with the need to limit recycle P loads to meet effluents limits created a glaring need for a sidestream struvite mitigation system. This plant upgrade also brought financial stress on the utility as the mandated effluent quality improvements carried a hefty capital price tag. As a result, the Provo WRP was keenly interested in finding ways to solve their struvite problems while limiting the impact to their capital budget. As the solids stream lead for the plant upgrade engineering team, Arcadis was tasked with identifying and evaluating advanced and novel sidestream P solutions that could address the full suite of operational and financial drivers. TECHNOLOGY SELECTION The technology selection included a competitive, performance based solicitation that considered a wide range of factors including P removal performance, capital cost, chemical addition, footprint requirements, and system O&M requirements. The resulting selected technology from this solicitation was a novel struvite precipitation process developed in Germany by Eliquo and dubbed the 'EloVac' process. This process is similar to other well-known struvite precipitation processes in that it aims to strategically raise sludge pH in a dedicated reactor, thereby increasing the solubility levels of struvite and creating struvite crystals that can be recovered or incorporated into the biosolids. The novel concept behind the EloVac technology is method by which the pH of the sludge is raised. While better known technologies utilize air stripping with blowers and diffusers and very tall reactors, the EloVac system utilizes a vacuum pump to raise sludge pH. This methods allows struvite precipitation to occur in a reactor that is the fraction of the size of traditional struvite precipitation technologies, requiring a residence time of only 10 minutes. It also eliminates the tall tank height and blower requirements involved with more traditional air stripping systems. The vacuum based EloVac is a compact system that can fit entirely on a 20'x10' prefabricated skid and has significantly reduced installation costs and footprint requirements. The main concern regarding this selection was the nascent nature of the technology, with only one other operating system in the world located in Lingen, Germany where it was developed. After technical due diligence the engineering team and the Provo WRP staff elected to move forward with a pilot system based on the cost and footprint benefits provided. While developed in Germany, the system is distributed in the US by Ovivo with its headquarters is located within the Salt Lake City area adjacent to Provo. Ovivo has had a successful history piloting its (then novel) linear motion mixer at the Provo WRP. To mitigate new technology risk, Ovivo and Provo negotiated an agreement to produce a full scale pilot capable of treating the entire plant sludge flow. The system is furnished under a 3 year lease agreement contingent on successful performance of the equipment during pilot testing, with Provo afforded the option to buy the equipment after the lease period. PILOT SYSTEM The containerized pilot system was installed at the plant site and started up in April 2021. The piloting was a collaborative effort between the engineering team (Arcadis), the equipment supplier (Ovivo), and the Provo WRP staff who performed the sample collection and laboratory analyses. The EloVac unit draws sludge from the plant's East Secondary Digester and returns struvite precipitated sludge to the West Secondary digester where it is then fed to dewatering. The major sampling points parameters measure are summarized in Table 1. RESULTS After an initial 10 day start-up period, the data collected for reactor performance showed immediate effectiveness for reducing sludge and centrate ortho-P concentrations below the target of 25 mg/L (as shown in Figure 1). This was not the entire picture, however, as Provo staff had initially kept dosing ferric to the centrate as a precaution during the pilot system start-up. This ferric dosing was discontinued in May (see Figure 2) which led to a build-up of struvite on the centrate pumps. This struvite formation was confounding as the ortho-P levels in the centrate remained low. After additional field investigations and analysis, it was determined that struvite was forming due to a combination of elevated pH levels occurring between the reactor effluent and centrate as well as overdosing of Mg. This combination of factors was driving the solubility product constant (Ksp) towards precipitation of struvite in the downstream centrate and not as crystals in the reactor as intended. Struvite precipitation on internal reactor piping was also observed during maintenance. Mitigating actions were taken including reducing the target Mg feed rate to the reactor as well as increasing the reactor level and vacuum pressure to drive more precipitation in the reactor and not the centrate. These adjustments appeared to be effective as the Provo staff were able to gradually reduce precautionary ferric dosing to the centrate and no new struvite formation was observed. CONCLUSIONS The based on full scale pilot testing results, the EloVac system was deemed to be effective in precipitation of struvite from the digested sludge. Throughout the pilot testing the effluent and centrate ortho-P concentrations remained at or below the target concentration of 25 mg/L. In September 2021 Provo staff reported that they had curtailed the precautionary ferric dosing to zero and that nuisance struvite had not been observed. This indicated that the system is working as intended to both prevent nuisance struvite and to prevent recycle of ortho-P back to the liquid stream. Provo WRP operators also noted that there have been observable benefits to dewatering performance in the form of higher cake solids and lower polymer dose. Currently the precipitated struvite crystals exit the plant via the biosolids but there are future plans to implement P recovery. Provo has accepted the equipment and is exploring options to permanently install it inside a building once the 3-year lease is completed. From a technology standpoint, this is an instance where a utility and engineering team took a chance to try something new. Backed by significant due diligence and a dedicated technology provider in Ovivo this willingness to test a novel technology resulted in considerable capital and operational savings. With these testing results in hand, sludge struvite management at this scale may be due for a paradigm shift as other plants of similar size can consider the EloVac technology when addressing their struvite issues.