Effects of iron and calcium carbonate on the variation and cycling of carbon source in integrated wastewater treatments

Highlights

• Dead algae as carbon source promoted microorganism activity in denitrification.

• Optimal dosage of Fe ion improved the removal performance.

• AP-CW realized optimal removals with 100 μM Fe³⁺, 50 μM iron and 0.2 mg/L CaCO₃.

Abstract

Iron and calcium carbonate were added in wastewater treatments as the adjusting agents to improve the contaminant removal performance and regulate the variation of carbon source in integrated treatments. At different temperatures, the addition of the adjusting agents obviously improved the nitrogen and phosphorous removals. TN and TP removals were respectively increased by 29.41% and 23.83% in AC-100 treatment under 1-day HRT. Carbon source from dead algae was supplied as green microbial carbon source and Fe²⁺ was supplied as carbon source surrogate. COD concentration was increased to 30 mg/L and above, so the problem of the shortage of carbon source was solved. Dead algae and Fe²⁺ as carbon source supplement or surrogate played significant role, which was proved by microbial community analysis. According to the denitrification performance in the treatments, dead algae as green microbial carbon source combined with iron and calcium carbonate was the optimal supplement carbon source in wastewater treatment.

Introduction

Ecological wastewater treatment is an efficient technology to remove contaminants from wastewater, such as constructed wetlands (CWs) (Zhao et al., 2016a). Nitrification and denitrification processes are common nitrogen removal pathways in CWs (Vymazal, 2010). Dissolved oxygen (DO) content (Nivala et al., 2013) and microbial carbon source (Ding et al., 2012, Ding et al., 2013) are the two key factors in the operation of CWs. Multiple phosphorus removal mechanisms are involved in CWs. Adsorption and biochemical reactions in substrate are the major phosphorus removal processes (Vymazal, 2010) and denitrification and phosphate removal become the hotspots in the field of wastewater treatment. CWs have been applied to treat wastewater due to their advantages, such as high space utilization efficiency (Brix and Arias, 2005), simple operation (Cui et al., 2012), and low construction and maintenance expenses (Stefanakis and Tsihrintzis, 2012, Bruch et al., 2011). In order to improve removal nitrogen and phosphates efficiency, it is necessary to develop an effective method to solve the shortage of carbon source in CWs treatment (Brix et al., 2001). In integrated treatments, such as algal pond combined with constructed wetlands (AP-CW), the removal performances of contaminants were improved by algae addition (Zhao et al., 2016a).

Iron has been widely used in ecologic wastewater treatments to increase the removal performance of nutrients because the physico-biochemical processes can be induced by its changeable chemical valence (Ma et al., 2014, Zhao et al., 2016b). Fe(II) can be easily oxidized into Fe(III) under aerobic conditions, while Fe(III) can be easily reduced to Fe(II) under reductive conditions, especially in CWs. There are various oxidation reduction zones in CWs. Additionally, iron(III) reducing bacteria are regarded as crucial mediators of C and N processes (Tan et al., 2006, Wang et al., 2009). Ferric iron can be reduced through respiration (as the electron acceptor) as well as fermentation (as an electron sink) under anaerobic conditions (Lin, 2006). Short-chain fatty acids under anaerobic conditions were oxidized and other anaerobes were produced, thereby affecting C mineralization (Bongoua-Devisme et al., 2012). Additionally, anaerobic NH₄⁺ oxidation is coupled to Fe³⁺ reduction with N₂, nitrite or nitrate as the end-product and may be an important pathway for nitrogen loss (Ding et al., 2014).

Calcium carbonate (CaCO₃) is a chemical compound composed of three main elements: carbon, oxygen, and calcium. It is a common substance in rocks, such as limestone. CaCO₃ exists in different forms with different specific stability. CaCO₃ is important in several wastewater treatment processes. In ecological treatment, CaCO₃ addition revises the diversity of microbial community and improves the microbial activity (Zhao et al., 2016b).

Nowadays, the study on carbon source in ecological wastewater treatments mainly focuses on organic carbon source. One method to improve the denitrification performance is to directly add organic matters, such as glucose and methanol (Santos et al., 2016). Another one is to recycle solid and liquid waste containing carbon, such as newspaper and plastics (Hao and Mi, 2011, Ansgar et al., 2016). In addition, the green carbon source generated with cheap natural materials, such as plants and leaves, may be also added into the wastewater treatments (Ding et al., 2013). Iron can be used as the donor supplier instead of organic matters to improve the denitrification performance in CWs (Song et al., 2016). However, to the best of our knowledge, the addition of iron and CaCO₃ as growth regulators in integrated treatments is not studied yet, especially in algae pond (AP) combined with CWs treatment. In the algae-microbial system, the regulating effects of iron and CaCO₃ on carbon source variation in algae pond (AP) combined with CWs treatment are still unknown. Microalgae can be used as green carbon source to increase the contaminant removal efficiencies during denitrification in AP-CW treatment (Zhao et al., 2016a), but the variation content and cycling condition of carbon source in wastewater treatment systems are still unknown. However, the improvement in the denitrification performance by algae as green carbon source in CWs was not reported.

The study aims to investigate the effects of iron and CaCO₃ on contaminant removal performance, explore the variation content and cycling condition of carbon source, and compare the denitrification performance in various integrated treatments utilizing algae as green carbon source or directly using traditional carbon source.