Dewatering Performance of Bio-P Plants.

Did you hear the Philadelphia Eagles won the Super Bowl?

Did you hear just how wrongly the pundits and odds makers had predicted the performance of the Eagles’ team, despite reams of statistics and analyses, and the input of experts of all kinds, whose careers are built on keen judgment? Just prior to the playoff game two weeks earlier,  NBC Sports wrote Eagles-Vikings NFC Championship Game predictions: “Despite the Eagles being the No. 1 seed and having home-field advantage, Minnesota is the favorite to advance and play the Super Bowl on their home field in Minneapolis.” But by Sunday night the story had become Underdog, huh? Eagles headed to Super Bowl

Even the betting experts, the people with money on the line, got the predictions really, really wrong. Top predictor R.J. White (“Anyone who has followed his advice has reaped big paydays”) was interviewed by CBS Sports in NFL playoffs Vegas expert picks and predictions: Eagles vs. Vikings.  He predicted: “The Vikings are favored by three points on the road after the line opened at 3.5.” But at the end of the game, the headline on CBS Sports was  Vikings vs. Eagles 2018 live results: Philly destroys Minnesota to reach Super Bowl 52

The scale of expert misjudgment was not corrected ahead of the Super Bowl, and the outcome was no less bad for the Las Vegas odds makers. The Patriots were favored by 4.5 points, and total points had settled at 48. By game’s end, Eagles were up by 8 points, and total points on the board were 74.

Ouch! The experts on both games were really, really off!

In the professional football industry, the entire system seems set up to support balanced competition among the franchise teams.  The rules of the game, the free agency system, salary caps, the pool of management talent, and the recruitment system are all managed to produce competitive teams. Yet, at the end of the season, the Cleveland Browns are 0-12 and the Philadelphia Eagles are 13-3. What factors account for such striking differences in performance?

The mystery of performance was brought home to me, ironically enough, by another surprising Minnesota performance.  This is not the performance of the Minnesota Vikings, but rather the performance of the wastewater plants operated by Metropolitan Council of the Twin Cities (Minneapolis/Saint Paul).  WEFTEC 2016 proceedings include this excellent, and candid, case study of poor dewatering performance (Investigations into Improving Dewaterability at a Bio-P/Anaerobic Digestion Plant). This agency struggles mightily with poor biosolids dewatering scores.

In a sense paralleling professional football, our field of expertise works with rules establishing a common playing field -- similar environmental regulations, similar financial incentives, the same basic treatment systems, the same available dewatering equipment, and the same access to engineering and operational talent. Yet, Metro’s plants are barely able to sustain 12% total solids cake coming from the belt filter pressing of mesophilic cake from advanced treatment for nutrient removal, and DC Water’s Blue Plains plant achieves upwards of 30% solids off of belt filter presses also treating biosolids from a nutrient removal system.  Go figure? What kind of analyses, statistics and professional judgments can you produce to explain the hugely different performances?

One explanation is the treatment process for accomplishing very low concentrations of phosphorus in effluent discharges. The treatment plants in the Twin Cities use enhanced biological nutrient removal of P, and DC Water uses ferric salts for a chemical precipitation of P.  The negative effect of the biological system on dewaterability compared to the chemical system is so striking that understanding the topic has its own research project at the Water Environment and Reuse Foundation.  This is  Unintended Consequences of Resource Recovery on Overall Plant Performance: Solving the Impacts on Dewaterability Properties (NTRY12R16).  WE&RF “entered into a contract with Bucknell University to determine if this phenomenon is more widespread, and to better understand and address dewaterability performance and its impact on Bio-P and P-recovery. This research is intended to fill the research gap and address the relevant fundamental mechanisms on dewaterability, effective plant operating parameters, and overall performance of different enhanced biological phosphorus removal treatments.”

When I contacted the WE&RF project principal investigator, Bucknell’s Matt Higgins, he included in his text message back to me the suggestion that I be in touch with Twin Cities’ George Sprouse. Sprouse shot back in his return text message this explanation: “We have two plants that have EBPR and anaerobic digestion and have the dewatering problems that Matt has been investigating. We've run pilot tests for several summers. Matt has seen those results, and we haven't found a cost-effective improvement method yet.” In follow up text messages, George indicated that cake solids are regularly south of 17% off of relatively new belt filter presses. Ouch! That performance is really, really off.

This stunning situation sent me to Google Scholar to search for ideas of how dewaterability of digested solids might be improved. Afterwards, I went through these options with Sprouse, and he has left few stones unturned.

The culprit in this story is extracellular polymeric substances released by anaerobic microbial communities. The journal article Extracellular polymeric substances and dewaterability of waste activated sludge during anaerobic digestion explained that “[t]he loosely bound extracellular polymeric substance (LB-EPS) content increased three-fold after 20 days of anaerobic digestion, and did not change significantly during the remaining 30 days…. The relationship analysis showed that only LB-EPS correlated with dewaterability of the sludge during anaerobic digestion.” So, treatment options need to rid sludges of the EPS.

One 2014 paper, Improving Dewaterability of Digested Sludge from EBPR Facilities, offered this listing of possible approaches: “Possible solutions may include adjusting the mono to divalent ion ratio by chemical addition or elutriation, cell lysis, metal salt addition, aeration, pH adjustment, or any combination thereof…. The addition of ferric showed the most promise for improving dewaterability and can simultaneously reduce the recycle phosphorus load at EBPR plant.”  The Metro research team looked at ferric, alum and chlorine. Only ferric had a clear beneficial effect.  But Metro found ferric effective only when combined with a period of tank storage of blended sludges with the iron.  

How about post anaerobic digestion with several days of aerobic treatment? This was one of the first trials Sprouse undertook with Dr. Higgin’s guidance.  The project Advanced anaerobic processes to enhance waste activated sludge stabilization had reported that “[T]he aerobic post-treatment after mesophilic digestion had a beneficial effect on dewaterability and stability of the digested sludge even if was with a reduction of the potential energy recovery.” Yet, Sprouse reported that at his plant the effect on dewaterability seemed negligible.

How about Ostara’s WASSTRIP removal of P in the secondary tanks? The paper Seeking to Understand and Address the Impacts of Biological Phosphorus Removal on Biosolids Dewatering postulated this approach: “stripping the phosphorus, magnesium and potassium from the waste activated sludge before digestion which would allow struvite recovery while also diverting potassium cations away from the solids processing train; and the concept of adding magnesium to the digested biosolids to satisfy the two-fold demand for magnesium in struvite formation and bioflocculation.” In concept, the process sounds plausible. Yet, trials in Minneapolis did not yield promising results.

How about zero valent iron? The authors of  Enhanced dewaterability of sewage sludge with zero-valent iron-activated persulfate oxidation system “observed that ZVI-S2O82 oxidation effectively improved sludge dewaterability. The optimal conditions to give preferable dewaterability were found when the molar ratio of ZVI/S2O82− was 5:1 and pH value was 3.0. The most important mechanism was proposed to be the degradation of extracellular polymeric substances (EPS) incorporated in sludge flocs and rupture of microbial cells.” A second study followed the same route:   Enhancing dewaterability of waste activated sludge by combined oxidative conditioning process with zero-valent iron and peroxymonosulfate.  But when Sprouse did this at Twin Cities, the approach did not move the dewatering performance.

Both peer-reviewed and gray literature postings on Google Scholar offer a variety of other technology options influencing dewaterability of biosolids.

One recently introduced process claims improved dewatering. CNP AirPrex introduces air and pH adjustment to alter dewaterability of the feedstock by first converting some of P-bearing polymeric substances to phosphate mineral, and thereby improving the cake solids in the cake going through dewatering equipment. In one report, Pilot-scale Evaluation of AirPrex® for Digestate Treatment, “In addition, this pretreatment resulted in a 15 to 20 percent reduction in polymer required for dewatering and a notable increase in dry cake solids leading to an overall 7 to 10 percent reduction in biosolids hauling requirements depending on the degree to which struvite is recovered as a separate fertilizer stream or commingled with the biosolids.”

How about a two-stage digestion with a pH adjustment to degrade the extracellular polymeric substances? In Application of acidic thermal treatment for one- and two-stage anaerobic digestion of sewage sludge, the authors explain: “The effectiveness of acidic thermal treatment (ATT) was examined… Therefore, VSS reduction was increased by 15–17%, when the ATT was combined in both one- and two-stage processes. In addition, the dewaterability of digested sludge was remarkably improved, and phosphate release was enhanced.“

High performance of anaerobic digestion seems to help.  After all, the deployment by DC Water of the Cambi Thermal Hydrolysis Process for pretreatment of sludges ahead of mesophilic digestion seems to be the process most distinguishing DC Water’s treatment from Twin Cities’ treatment.  Cambi THP takes digestion to a further point of volatile solids removal than seems possible with mesophilic digestion alone. The authors describe the benefits of Cambi on dewaterability in Optimization of belt filter press dewatering of thermally hydrolyzed biosolids: A Blue Plains case study on balancing desired cake solids with filtrate quality. They write: “THP treatment increased the cake TS by 88% for the mixed sludge and 19% for the anaerobic digested sludge. The study also found that the cake TS was negatively correlated with bound water (R2= 0.97) and EPS (R2=0.76) in the sludge. This suggested that the improvement of cake TS were mostly achieved by the THP causing release of bound water from the sludge."

As an alternative to thermal hydrolysis as a pretreatment to digestion, Cambi is introducing post digestion thermal hydrolysis. This is the focus of the Cambi SolidStream system, with one reference facility under study and other installations planned. Post-anaerobic digestion thermal hydrolysis of sewage sludge and food waste: Effect on methane yields, dewaterability and solids reduction showed promising results: “Results from thermogravimetric analysis showed an expected increase in maximum TS content of dewatered digestate cake from 34% up to 46% for the SSFW digestate cake, and from 17% up to 43% in the sludge digestate cake, after the PAD thermal hydrolysis process (PAD-THP). The increased dewatering alone accounts for a reduction in wet mass of cake leaving the plant of 60% in the case of sludge digestate cake.” More details on how this might be deployed is given in a second paper Cambi SolidStream® : Thermal Hydrolysis as a pre-treatment for dewatering to further reduce operating costs. Either as a pre-treatment or as a post-treatment, thermal hydrolysis may work in transforming bio-P sludges into a dewaterable feedstock to conventional dewatering equipment.

Aside from thermal hydrolysis, the science literature holds out some other hope for improving dewaterability.

How about oxidizing the polymeric substances before dewatering? Fenton’s reagent came up in a couple of studies. This is the reaction that involves hydrogen peroxide as an oxidizer and ferrous iron as a catalyst. One paper took it a step further with a surfactant to help capture suspended polymers: Mechanism and Parameter Optimization of Fenton’s Reagent Integrated with Surfactant Pretreatment to Improve Sludge Dewaterability.  According to the authors: “Analysis of the reaction mechanism showed that Fenton oxidation effectively decomposed extracellular polymeric substance (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), into dissolved organics, such as proteins and polysaccharides. [And,} DDBAC (the surfactant) further released the bound water through solubilization of TB-EPS and LB-EPS after the Fenton reaction.”

How about nano-scale aluminum powder? One researcher has been advocating for this mineral additive and has sought a patent on a nanoscale aluminum powder. Its proponent (Use of Nanoparticles for Reduction of Odorant Production and Improvements in Dewaterability of Biosolids) offers some promising results: “Depending upon the source of sludge, between 20% and 60% reduction in polymer dose was observed. This was accompanied by a 10–20% increase in cake solids.” The author, who has submitted for a patent for this application (Use of nanoscale particles (nanoadditives) with cationic polymers to enhance conditioning and dewatering of sludge),  includes in his paper this caveat: “additional studies are required to understand the mechanisms and to improve nanoparticles composition and configuration for cost effective biosolids treatment using nanoparticles.”

The incentives are abundant out there for finding a solution. We have the tools and talents to test, analyze, explore, and correct our operations, and the WE&RF research project is a great focal point for our own pundits and odds makers to predict a game winner.  I believe one secret to the success of the Philadelphia Eagles this year was the solid and cohesive team spirit in the locker room and on the field.  We are seeing that same team spirit in our wastewater community for solving this challenge to the dewatering performance of Bio-P plants. Though it will certainly not yield a million-person street parade, someday we will rightfully celebrate a SOLID BIOSOLIDS PERFORMANCE!