Impact of Polymer Dosing on Thermal Hydrolysis Return Liquor


  • Nandita Ahuja, John T. Novak, Trung Le - Virginia Polytechnic Institute and State University
  • Matthew J. Higgins - Bucknell University
  • Sudhir N. Murthy - District of Columbia Water and Sewer Authority

Research and technological advancements in the field of biosolids treatment/sludge management have gained momentum in the last few decades. This growing interest is primarily driven by stricter regulations being passed for disposal of treated sludge, increasing hauling costs associated with its disposal, decreasing land availability and prioritizing of energy efficiency (Odegaard 2004, Pérez-Elvira et al. 2006). Pre-treatment of sludge at high pressure and high temperature prior to mesophilic anaerobic digestion (MAD), called the thermal hydrolysis process (THP) is one such technological advancement being adopted by several wastewater treatment plants across Europe and North America. THP as a pre-treatment to MAD has been reported to achieve better cake dewaterability, increased methane production, increased digester loading rates and production of Class A bio-solids (Carrère et al. 2010, Haug et al. 1978, Kepp et al. 2000, Neyens and Baeyens 2003). While there are numerous merits associated with adoption of this process, it has also led to increasing concerns with regards to the downstream impact of the THP recycle stream.

The dewatered side stream/recycle stream/centrate from the THP (after MAD), also referred to as the THP return liquor, contains a higher concentration of ammonium, dissolved organic nitrogen (DON) and UV254 quenching substances than that from a conventional MAD process (Constantine 2006, Dwyer et al. 2008, Figdore et al. 2011, Phothilangka 2008, Wilson et al. 2011). When recycled into the main wastewater stream this concentrated return liquor has the potential to impact nitrification and total nitrogen removal achieved by the plant (Constantine 2006). While mature technologies are available to reduce the inorganic nitrogen (DIN) in the effluent, the organic nitrogen recycled through the return liquor is mostly refractory in nature and is not easily removed by the biological nutrient process (BNR) and can consequently end up in the final effluent from the plant (Dwyer et al. 2008, Phothilangka 2008).

Thus, the objective of this research is to understand the impact of the polymer dosing in the sludge-dewatering stage, on the DON and formation of UV quenching substances in the THP return liquor. Findings of this research would be helpful in further optimizing the THP to minimize the DON and the formation of UV quenching compounds being contributed by the side-stream to the downstream processes.

THP Return Liquor
The samples were received from the THP pre-treated anaerobic sludge treatment pilot plant at the Blue Plains Advanced Wastewater Treatment Plant, Washington DC, USA. The thermal hydrolysis reactor at the pilot plant was operated at a temperature of 150°C and 170°C. Thermally hydrolyzed sludge for these two operating two operating temperatures was then transported to the laboratory at Bucknell University, Pennsylvania, USA where it was anaerobically digested at 38°C for an SRT of 15 days. This was followed by transportation of the digested sludge to the laboratory at Virginia Tech, Virginia, USA.

Polymer Conditioning
This sludge was conditioned with polymer Clarifloc-3258 followed by dewatering using a funnel fitted with a belt filter cloth. Polymer doses used in this study were 6.8 mg/kg (15 lb/ton), 9.1 mg/kg (20 lb/ton), 11.3 mg/kg (25 lb/ton), 13.6 mg/kg (30 lb/ton) and 15.9 mg/kg (35 lb/ton). Polymer dose is expressed as dry weight of polymer per unit dry weight of sludge. Filtrate from the dewatering process was then analyzed for this study.

Fractionation based on molecular size and hydrophobicity
The centrate was passed through 0.45μm membrane (Millipore, Billerica, MA), to separate the dissolved solids fraction from the suspended solids fraction followed by sequential filtration through ultrafiltration (UF) membrane disks (Millipore, Billerica, MA) of molecular weight cut-off (MWCO) 100k Da, 3k Da and 1k Da.

The samples were characterized into Humic Acids (HA), Fulvic Acids (FA) and Hydrophilic Acids (Hpi) based on the methods described by (Leenheer 1981, Thurman and Malcolm 1981).

Organic nitrogen in the samples was estimated by subtracting ammonium from total kjeldahl nitrogen (TKN). TKN was measured by semi micro-Kjeldahl Method per Standard Method 4500-C (APHA et al. 2012). Ammonium was determined by the salicylate method (Hach, Loveland, CO) and measured using a spectrophotometer (DR 2800, Hach, Loveland, CO).

Total Organic Carbon (DOC) was analyzed using a high temperature combustion DOC analyzer (Shimadzu TOC- 5000A, Japan). The UV absorbance at 254 nm (UV254 abs) was measured with a spectrophotometer (Beckman Coulter, Brea, CA).
HA-DON, FA-DON, Hpi-DON refers to DON in humic acids fraction, fulvic acids fraction and hydrophilic acids fraction respectively. Similarly, HA-DOC, FA-DOC, Hpi-DOC refers to DOC in humic acids fraction, fulvic acids fraction and hydrophilic acids fraction respectively.

• Increasing the polymer dose results in a decrease in DON in the return liquor up to polymer dose of 13.6 mg/kg followed by an increase in DON when the polymer dose is increased to 15.9 mg/kg.
• A significant decrease of 55% and 36% in DON for fraction <1k Da at 150°C and 170°C respectively is observed when the polymer dose is increased from 9.1 mg/kg to 11.3 mg/kg.
• Trends followed by DOC at operating temperature of 150°C and 170°C are similar to that observed for DON for polymer dose between 6.8 mg/kg-13.6 mg/kg.
• Upon increasing the polymer dose to 15.9 mg/kg there is a decrease observed in DOC at 150°C and 170°C, as opposed to the increase in DON observed for the same samples.
• The UV254 abs generally decreases with increase in polymer dose for all polymer doses

Increasing polymer dose can significantly reduce DON, DOC and UV254 abs in the THP return liquor for both operating temperatures of 150°C and 170°C, including the fraction <1k Da in size. The increase in DON when the polymer dose is increased beyond 13.6 mg/kg is likely due to the availability of excess polymer, which does not bind to organic compounds. The organic nitrogen present in this excess polymer itself contributes to the DON in the return liquor resulting in an overall increase in DON observed at higher polymer doses.

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