Impact of Ozonation and Electrocoagulation on Refractory Nutrients Generated in Thermal Hydrolysis

Authors:

  • Gregory Pace, Wendell Khunjar, Phill Yi, Stephanie Ishii, Eric Dole, Marc Santos - Hazen and Sawyer
  • Robert Sharp - Manhattan College, NY
  • Eukalak Khan, Ruchi Joshi - North Dakota State University, ND

Figure 1. Removal of DON and sCOD from THP centrate. A – Dose response for Ozone treatment, B – Influence of reaction time during Electrocoagulation

Figure 2. Influence of Ozone and Electrocoagulation on THP centrate excitation-emission matrix spectroscopy (EEMs) characteristics

Summary of Research:

  • Thermal hydrolysis of municipal solids can result in the production of non-biodegradable organic nutrient fractions that are not effectively treated with conventional sidestream and mainstream treatment processes.
  • Utilities managing treatment facilities that need to meet enhanced nutrient and organic removal limits (TN < 3 mg/L and TP < 0.18 mg/L; COD < 10 mg/L) will need to consider additional treatment for the non-biodegradable organic nutrient fractions.
  • In this study, sidestream treatment of the non-biodegradable nutrient pool was tested using ozone or electrocoagulation.
  • Results from experiments with ozone indicated: Ozone treatment was effective at reducing sCOD (5 to 7%), DON (60 to 95%) and UV254 (3 to 6%) in sidestream; Reduction of sCOD, DON and UV254 was proportional to ozone doses tested; NH3 production can be expected when ozonation of the sidestream is performed.
  • Results from experiments with electrocoagulation indicated: Electrocoagulation can achieve reduction of sCOD (12 to 16%), DON (up to 95%) and UV254 (17 to 29%) in the sidestream; No significant benefit to removal was observed with increased reaction time
  • Full-scale implementation of these options at ENR WRRFs has the following implications: Ozone treatment would cost approximately $6.60/lb DON removed (operating cost only); Electrocoagulation treatment would cost approximately $7.60/lb DON removed (operating cost only); Both ozonation and electrocoagulation would require pre-treatment of the sidestream to remove solids.
  • Project Motivation:

    Thermal hydrolysis of municipal solids represents an opportunity for WRRFs to increase the rate of solid hydrolysis, increase the extent of volatile solids reduction and boost biogas production during anaerobic digestion. Alongside this increased rate and extent of solids hydrolysis is a concomitant increase in nitrogen (N) and phosphorus (P) solubilization. A subset of this released nutrient pool is recalcitrant to biodegradation during anaerobic or aerobic processes (Dwyer et al., 2008) and therefore passes through sidestream and mainstream biological nutrient removal processes untransformed. This recalcitrant nutrient pool can increase the total effluent nitrogen and phosphorus discharged from WRRFs.

    ENR WRRFs have several options for addressing this increase in effluent nutrient load. Facilities that have a mass allocation can try to offset this pool by purchasing additional chemicals to remove more readily accessible nutrient fractions (e.g., nitrate and/or nitrite and ortho-phosphorus). Alternatively, advanced treatment processes in the mainstream like reverse osmosis, advanced oxidation and granular activated carbon can be considered; however, these advanced treatment processes require significant cost (capital and operating) expenditures and footprint.

    In this work, we investigate alternate treatment strategies for removing the non-biodegradable nutrient pool generated during thermal hydrolysis. We tested sidestream ozonation and electrocoagulation to determine whether these processes can cost effectively remove the non-biodegradable nutrient pool.

    Approach:

    Centrate from a facility performing thermal hydrolysis and mesophilic anaerobic digestion was obtained. This centrate was characterized for total nitrogen, total khejdahl nitrogen, ammonia, nitrate, nitrite, orthophosphate, UV254 and chemical oxygen demand via standard methods (Eaton, Clesceri et al. 1995) as well as excitation-emission matrix spectroscopy (EEMs) and biodegradable DON (Khan et al., 2009). Once characterized, aliquot samples were treated at varying doses of ozone in controlled bench-scale reactors or separately at different reaction times in an electrocoagulation reactor.

    Ozone solution was generated at lab scale and combined with THP centrate at doses of 0, 1, 3, 5 and 8 mgO3/L. Separately, THP centrate was processed through a bench-scale electrocoagulation reactor for 1 and 4 minutes respectively. Treated centrate samples were then characterized for nutrient and organic fractions as described previously.

    Results from the testing were then used to develop a concept design of implementing sidestream treatment with ozone or electrocoagulation at a full-scale ENR WRRF. Capital and operating costs for the options were estimated in line with AACE Class 5.

    Results and Discussion:

    Results from experiments with ozone indicated:

  • Ozone treatment was effective at reducing sCOD (5 to 7%), DON (60 to 95%) and UV254 (3 to 6%) in sidestream.
  • Reduction of sCOD, DON and UV254 was proportional to ozone doses tested.
  • NH3 production can be expected when ozonation of the sidestream is performed.
  • Results from experiments with electrocoagulation indicated:

  • Electrocoagulation can achieve reduction of sCOD (12 to 16%), DON (up to 95%) and UV254 (17 to 29%) in the sidestream.
  • No significant benefit to removal was observed with increased reaction time.
  • Full-scale implementation of these options at ENR WRRFs has the following implications:

  • Ozone treatment would cost approximately $6.60/lb DON removed (operating cost only)
  • Electrocoagulation treatment would cost approximately $7.60/lb DON removed (operating cost only).
  • Both ozonation and electrocoagulation would require pre-treatment of the sidestream to remove solids.
  • For more information, please contact the author at gpace@hazenandsawyer.com.

    Hear about new publications with our email newsletter

    We will never share your details with anyone else.

    Horizons

    Newsletter Newsletter

    Horizons Summer 2018 (pdf)

    Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.

    View previous issues »