Intermittent cyclic process for enhanced biological nutrient removal treating combined chemical laboratory wastewater

Abstract

The aim of this work was to assess the efficacy for simultaneous enhanced removal of nitrogen and phosphorus including organics treating combined wastewater generated from a chemical laboratory using a bench-scale Intermittent Cyclic Process Bio-reactor (ICPBR). The performance efficacy indicated that the ICPBR system with solid retention time of 15 days achieved optimum efficiency with an overall removal of ammonia nitrogen (NH₄–N), phosphorus (PO₄–P), and chemical oxygen demand (COD) in the range, 83–92%, 74–93%, and 90–96%, respectively.

Introduction

Nutrient removal from wastewater has become an important concern because of increased occurrence of eutrophication in surface water bodies. Nitrogen and phosphorus both are responsible for eutrophication. Nutrient removal technologies include physical, chemical and biological methods. The preference for a biological process for nutrient removal as compared to physico-chemical methods is the low treatment cost involved (Van Dongen et al., 2001). However, the conventional biological wastewater treatment systems like activated sludge process, trickling filter and stabilization ponds do not facilitate enhanced nutrient removal. An application of an appropriate technology for enhanced nutrient removal is necessary especially for wastewaters being finally discharged into water bodies (Mace and Mata-Alvarez, 2002, Dapena-Mora et al., 2004).

The removal of nitrogen through biological means is accomplished by nitrification followed by de-nitrification (Sousa and Foresti, 1996, Carvantes et al., 2001). The first step nitrification is the aerobic oxidation of ammonia to nitrite, and then to nitrate. The process is performed by nitrifying bacteria, which get their energy from the oxidation of these nitrogen compounds (Wiesmann, 1994). The following stage is de-nitrification, where nitrate (formed in the nitrification step) is anoxically transformed into nitrite, then to nitrous oxide, nitric oxide, and finally to gaseous nitrogen. Denitrifying microorganisms are hetrotrophic, and in anoxic condition use nitrate or nitrite as the final electron acceptor (Zeng et al., 2003, Lucas et al., 2005).

The biological phosphorus removal is basically achieved through modification in the activated sludge process. A combination of anaerobic and aerobic cyclic phases is implemented to have excess phosphorus-accumulating organisms (PAOs). PAOs are in competition with the denitrifying organism for the easily degradable substrate like acetate, propionate, etc. (Bernardes and Klapwijk, 1996). The PAO are capable of hydrolyzing inorganic polyphosphate stored in the cells to provide energy for the metabolism of simple organic compounds (short-chain volatile fatty acids). The volatile acids are converted to cellular stored products (poly-hydroxyalkalanoate: PHA, poly-hydroxybutyrate: PHB). The PAOs in the anaerobic phase store PHB that is a degradation product of poly-P by utilizing carbon source, and they release orthophosphate in to the bulk liquid. Subsequently, in the aerobic phase, the stored products (PHB) are metabolized to generate energy for the growth and synthesis of glycogen and poly-phosphate resulting in the uptake of phosphate from wastewater (Smolders et al., 1996, Kargi et al., 2005).

Intermittent cyclic bioprocess is a modification of an activated sludge process tailored to provide enhanced nutrient removal through sequencing of the unit processes (aerobic/anaerobic/anoxic). The process has several advantages over conventional continuous flow system comprising enhanced nitrogen and phosphorus removals, minimal space requirement, reduction in operating costs, less sludge bulking and the control over micro-organisms selection, including operational flexibility and ease of operation (Hamamoto et al., 1997, Obaja et al., 2005, Zhang et al., 2005). Nitrogen and phosphorus removal can be accomplished in a single tank ICPBR, provided, operating conditions are properly selected to introduce anaerobic, aerobic and anoxic reactions during a cycle without addition of separate reactors, recycling lines or clarifiers (Altinbas, 2001, Obaja et al., 2003, Kargi et al., 2005).

In an ICPBR system, the cycle includes fill, react, settle, decant and idle phases. Based on the degree of removal desired, the operation can be varied by altering the time period of the treatment phases (Kargi and Uygur, 2004). During the operation of the system under different cyclic phases, the oxygen uptake rate, and F/M ratio are constantly changing. The period, when the F/M ratio and the oxygen uptake rate are high is a “feast” period, and when the F/M ratio and uptake rate are low is a “famine” period (Norcross, 1992). At low organic loading with partial or lower nitrification, phosphorus removal is achievable (Furumai et al., 1999). A typical ICPBR design, with an average influent concentration of 5–15 mg NH₄–N/l and 2–4 mg PO₄–P/l, can achieve biochemical oxygen demand (BOD) and suspended solids (SS) concentration of less than 10 mg/l, nitrification up to 1–2 mg/l NH₄–N, and de-nitrification and phosphorus removal less than 1.0 mg/l, without addition of chemicals (Surumpalli et al., 1997). Minor modification in the system design can achieve improved phosphorus removal efficiency is also reported (Imura et al., 1993).

The objective of the study was to investigate the performance of an ICPBR for enhanced nutrient removal from a chemical laboratory wastewater mixed with township domestic sewage having high influent nitrogen and phosphorus concentrations.