Impacts of hydraulic retention time on granule behaviour and reactor activity during hydrocarbon degradation in aerobic granular reactors (AGRs) with phytotoxicity analysis

Highlights

• Bigger granules in 12 h HRT and 90% oil removal in 48 h HRT.

• Hydrocarbons converted into fatty acids and undergone β-oxidation.

• About 2–20% nitrogen in complete nitrification and rest in biomass growth.

• High biomass concentration in 12 h HRT provided maximum reactor activity.

• Effluent of 48 h HRT provided maximum germination and chlorophyll in plants.

Abstract

The paper is about impacts of hydraulic retention time (HRT) on granule characteristics, reactor activity, hydrocarbon removal efficiency and removal mechanism while treating emulsified diesel wastewater in aerobic granular reactors (AGRs). AGR with shortest HRT (12 h) achieved maximum 4.72 ± 0.05 mm granule size and extracellular polymeric substances (EPS) of 400.31 ± 0.01 mg/g volatile suspended solids (VSS) with 68.85 ± 0.06% of hydrocarbon removal. Short HRT (12 h) provided maximum biomass concentration (6.38 ± 0.05 g/L) to achieve maximum biomass activity of 8.33 ± 0.21 mg COD/mg VSS.day among the AGRs. But longest HRT (48 h) played major role providing longer reaction time for 90.31 ± 0.26% hydrocarbon removal. Degradation of short and long chain alkanes (C₆–C_7, C₉–C_10, C₁₁–C_13, C₁₅–C_18, C₂₇) were observed in the AGRs which further produced fatty acids as metabolites. In steady state COD removal rate varied between 71- 90% which was completely depended on HRT values and hydrocarbon removal efficiency of the reactors. Nitrogen balance suggested that maximum nitrogen was utilized in biomass growth and about 2–20% nitrification occurred in the AGRs. About 24–48 h HRT with 0.125–0.25 kg/m³.day hydrocarbon loadings providing below 77 mg/L effluent hydrocarbon was recommended for successful AGR treatment to avoid phytotoxicity after disposal.

Introduction

Since last three decades, aerobic granulation technology has been extensively studied in industrial wastewater treatment (Reddy et al., 2017; Tomar and Chakraborty, 2018, Tomar and Chakraborty, 2018; Ghosh and Chakraborty, 2019). Aerobic granules are fluffy flocs of densely packed sludge microbes having compact structure, good settling properties, tolerance to shock loading with simultaneous nutrient and toxic compound removal capacity which can be operated with low cost (Khan et al., 2013).

Massive production and poor biodegradability of the hydrocarbon rich oily wastewater are the major problems for designing an efficient biological treatment process. Hydrocarbon rich wastewater including petroleum, palm oil mill (POME), coal gasification and hypersaline oily effluents have been treated in AGRs so far (Chen et al., 2019; Zhang et al., 2011; Gobi et al., 2011; Milia et al., 2016; Corsino et al., 2015). AGR was successfully implemented for POME treatment which achieved complete COD removal by adsorption and biodegradation in aerobic granules (Gobi et al., 2011). Zhang et al., 2011 noticed poor granule performance with increasing proportion of real petrochemical wastewater inside AGR and finally achieved 89% COD and 67% NH₄⁺-N removal in co-metabolism of sodium propionate and petroleum wastewater. AGR was capable of hydrocarbon removal in simultaneous presence of low hydrocarbon and high chloride and it caused absence of nitification due to lack of autotrophic biomass (Corsino et al., 2015). Both Milia et al. (2016) and Chen et al. (2019) faced deterioration in nitrogen removal performances of the granules due to absence of dentrificans while treating coal gasification and petroleum wastewater. In our recent study, synthetic hydrocarbon rich oily wastewater was treated in AGRs using different inoculum at 16 h HRT and almost 70% hydrocarbon removal was observed at 320 mg/L of influent hydrocarbon concentration (Ghosh and Chakraborty, 2019).

HRT plays an important role in reactor performance. It influences the average contact time between a soluble substrate and aerobic granules which further controls the treatment efficiency of the AGR. Pan et al. (2004) optimized 2–12 h HRT for stable granulation and Rosman et al. (2014) observed that low HRT enhanced COD and nitrogen removal in rubber wastewater treatment. Tomar and Chakraborty, 2018 reported that granule size and EPS content were inversely proportional to HRT whereas pollutant removal was independent of the HRT while treating phenol and ammonia in AGRs. Role of HRT on aerobic granules and pollutant removal efficiencies are mostly reported on simple wastewater containing acetate (Moy et al., 2002; Ghangrekar et al., 2005) rather than having hydrocarbon and nitrogen. Literatures are limited on hydrocarbon degradation pathway by aerobic granular sludge, nitrogen removal in presence of hydrocarbons and on nitrogen balance. Moreover, in agricultural irrigation of oily wastewater, hydrocarbon toxicity can affect crop production contaminating the entire food chain (Yu et al., 2017). Hence, phyto-toxicity study is essential to determine the environmental viability of the AGR treated effluent.

This paper mainly focuses on the effects of changing HRTs on granule characteristics, reactor activity, COD removal efficiencies with probable hydrocarbon degradation mechanism and pathway and detailed nitrogen balancing during synthetic hydrocarbon wastewater treatment in AGRs. The study further determined suitable range of HRT and influent loadings for AGR operation to achieve complete hydrocarbon removal. Phytotoxic effect of treated hydrocarbon wastewater was also checked on two largely cultivated legume plants Cicer arietinum and Vigna radiata.