Optimization of Nitrogen Removal from Anaerobically-pretreated Swine Wastewater (APTSW) in Intermittent Aeration Reactors

Abstract

The swine industry is a large part of NC's economy generating over $2 billion in annual profit in the state. With a large hog production in NC, disposal of generated swine wastewater has entered the forefront of environmental concerns. Swine wastewater contains high levels of chemical oxygen demand (COD) and nutrients including nitrogen (N), phosphorus (P), and minerals. The most common treatment of swine wastewater in the southeast U.S. is the use of an anaerobic lagoon, which is an economical and practical way to provide reduction of COD through an anaerobic treatment and long-term storage. A lagoon, though easily maintained, does not provide significant reduction N and P which, if left untreated can lead to problems in water sources.

In this study an intermittent aeration (IA) reactor was used to remove nitrogen (N) from an influent of anaerobically pretreated swine wastewater (APTSW) (hog lagoon waste). The COD/TKN ratio, aeration to non-aeration ratio (ANA), and cycle time were varied to investigate their influence on N removal from IA reactors. The results show that removal of N was high for all ANA ratios and cycle times tested, 1h:1h (A), 1h:3h (B), 0.5h:1.5h (C), 0.5h:2.0h (D), and 1h:4h (E). It was evident that increasing the COD/TKN ratio did affect denitrification activity because effluent NO3-N concentration in the effluent of the reactors tested decreased. A yearly average reduction of over 80% TN and 86% NH3-N was obtained from all the reactors. Therefore there was no significant difference in removal of N from reactors with lower daily aeration fractions (0.2), showing that reactor E (0.2 and 1h:4h ANA) is the best choice when selecting an IA systems.

Since all reactors operated with the similar N removal efficiencies, environmental parameters were monitored to establish differences within the reactors. Oxidation reduction potential (ORP), dissolved oxygen (DO), pH, and nitrogen species profile were measured through an ANA cycle. The profiles of ORP, DO, and pH established whether nitrification or denitrification N reactions were predominate. Nitrogen species profiles showed which N constituents were prevalent in nitrification and denitrification reactions. Rates for nitrification and denitrification were calculated based on N species profile.