Hybrid zeolite-based ion-exchange and sulfur oxidizing denitrification for advanced slaughterhouse wastewater treatment

ABSTRACT

The discharge of slaughterhouse wastewater (SWW) is increasing and its wastewater has to be treated thoroughly to avoid the eutrophication. The hybrid zeolite-based ion-exchange and sulfur autotrophic denitrification (IX-AD) process was developed to advanced treat SWW after traditional secondary biological process. Compared with traditional sulfur oxidizing denitrification (SOD), this study found that IX-AD column showed: (1) stronger ability to resist NO₃⁻ pollution load, (2) lower SO₄²⁻ productivity, and (3) higher microbial diversity and richness. Liaoning zeolites addition guaranteed not only the standard discharge of NH₄⁺-N, but also the denitrification performance and effluent TN. Especially, when the ahead secondary biological treatment process run at the ultra-high load, NO₃⁻-N removal efficiency for IX-AD column was still ~100%, whereas only 64.2% for control SOD column. The corresponding average effluent TN concentrations for IX-AD and SOD columns were 5.89 and 65.55 mg/L, respectively. Therefore, IX-AD is a promising technology for advanced SWW treatment and should be widely researched and popularized.

Introduction

As the development of economy and the improvement of living standard, the demand for meat products is increasing greatly. The discharge of slaughterhouse wastewater (SWW) is increasing because of the rapid development of slaughtering industry. SWW was listed as one of the most harmful industrial wastewaters in agriculture and food industry by the united states environmental protection agency (USEPA) in 2004 (USEPA, 2004). High contents of organic, nitrogen and phosphorus in SWW have to be treated thoroughly before its discharged to the receiving water area to avoid the outbreak of eutrophication or/and pollution of groundwater.

Biological treatment method is one of the most economical and effective methods for SWW treatment (Wahaab and EI-Awady, 1999; Bustillo-Lecompte and Mehrvar, 2015). The conventional biological SWW treatment processes can effectively remove COD and NH₄⁺, but the eluent total nitrogen (TN) is difficult to control, which is mainly consist of NO₃⁻ and a small amount of NO₂⁻ by-products and the residual NH₄⁺ (Del Pozo and Diez, 2005; Tong et al., 2020). Further advanced treatment is usually required to control effluent TN and ensure that it satisfy the upcoming more stringent SWW effluent standard (for example in China).

Denitrification is an effective way to remove NO₃⁻ and control effluent TN. In heterotrophic denitrification systems, the organic substrate must carefully match to the NO₃⁻ loading rate. However, the remainder COD from secondary biological treatment process is low, where the substances are not conducive to microbial utilization (Zhao et al., 2012). Incomplete denitrification would occur to accumulated lots of by-products, when the organic substrate is insufficient (Zhou et al., 2011). Sulfur oxidizing denitrifying bacterial eliminates this problem by using elemental sulfur as an electron donor according to the following equation (Batchelor and Lawrence, 1978):

1.10S⁰ + 0.40CO₂ + NO₃⁻ + 0.76H₂O + 0.08NH₄⁺ →0.08C₅H₇O₂N + 0.50N₂ + 1.10SO₄²⁻ + 1.28H⁺

The lower growth rates autotrophic bacteria also result in low excess biomass production (Zhou et al., 2011) and decreased maintenance requirements for reactors/columns backwashing (Tong et al., 2017a). Prior studies have shown that heterotrophic and autotrophic denitrification processes can be combined and promoted mixotrophic metabolism to increase denitrification rates, reduce SO₄²⁻ production and alkalinity requirements (Krayzelova et al., 2014; Tong et al., 2017b; Jin et al., 2019).

In addition, residual NH₄⁺-N after conventional biological treatment processes has to be advanced treated, when TN in SWW is too high to leave lots of NH₄⁺-N. NH₄⁺ ion-exchange materials can be used as the biofilm carriers to control effluent NH₄⁺, while denitrification was used to removal NO₃⁻. When influent NH₄⁺ higher than the sulfur oxidizing denitrification (SOD) requirement, the above materials can temporarily ion-exchange NH₄⁺ with Na⁺, Ca²⁺, Mg²⁺ (Aponte-Morales et al., 2016). Reverse ion exchange can occur to supply NH₄⁺ as the nutrient when influent NH₄⁺was lower than SOD requirement. Hence, this study introduced a kind of zeolite as ion-exchange filter media to regulate effluent NH₄⁺ and enhance denitrifying bacteria activity to advanced remove TN from secondary biological treated SWW.

Zeolites are mineral aluminosilicates with a tetrahedral ring framework and extraframework cations with cation exchange properties (Hedstrom, 2001). Natural zeolite materials have been used for removal of NH₄⁺ in a number of wastewater treatment applications. For example, Huang et al. (2014) adopted modified zeolite to remove NH₄⁺ from simulated swine wastewater, finding that (1) NH₄⁺ can be effectively removed and (2) the presence of ions in zeolites had a positive influence on the removal of PO₄³⁻. However, concentrated salt solutions regenerated would present a disposal problem. Prior studies showed that NH₄⁺ saturated zeolite can be regenerated by biological process without NaCl addition (Semmens et al., 1977; Aponte-Morales et al., 2016). Microorganisms use the NH₄⁺ in solution that is in equilibrium with NH₄⁺ desorption from zeolite. Cations present in SWW (Tong et al., 2020) help to desorb NH₄⁺, therefore supplemental salt addition would be not necessary. Hence, adding zeolites as ion-exchange filter in advanced SWW treatment process would help to control effluent NH₄⁺.

Therefore, the overall goal of this research was to develop a low cost and efficient advanced TN removal process based on SOD and ion exchange processes that can be applied in SWW treatment systems. The specific objectives were to: (1) determine the NH₄⁺ ion-exchange and PO₄³⁻, COD and NO₃⁻ adsorptive capacities of Liaoning zeolites (origin: Liaoning, China), (2) characterize the potential of Liaoning zeolites used as both ion-exchange filters and biofilm carriers, (3) investigate the TN removal efficiency through the developed hybrid zeolite-based ion-exchange and sulfur oxidizing denitrification (IX-AD) process in column study under steady and variable loading conditions, (4) evaluate the contributions of Liaoning zeolite addition in IX-AD column on the resistance to loads fluctuation and the increase of denitrifying microbial diversity and richness and (5) evaluate the advanced treatment capacity of IX-AD column for the secondary effluent of actual SWW.