Influence of biodegradable polyurethane foam on biocoenosis and sludge activity in reactors simulating low-load wastewater treatments

Highlights

• Fully aliphatic PUR foam is used as biofilm carrier and substrate source.

• Wastewater plant simulation shows a positive impact of the added foam.

• Hydrophilicity and open pore structure ensure successful biofilm attachment and immobilization.

Abstract

Hydrophilic and fully aliphatic polyurethane (PUR) foam exhibiting a highly open cellular structure was used as a biodegradable biofilm carrier that would serve simultaneously as a nutrient source. A one-month laboratory testing in a reactor simulating domestic wastewater treatment plants was conducted based on chemical and hydrobiological analyses as well as evaluation of microorganism development and enzymatic activity. It was found that PUR foam can successfully serve as a carrier for microorganisms and simultaneously can be biodegraded when the supply of nutrition is limited.

Introduction

Polyurethane (PUR) foams exhibit widespread use in many industrial sectors and applications due to their fast and simple preparation, tuneable cellular morphology and versatile end-use properties [1]. More particularly, the high specific surface area and high open-pore content of PUR foams make them suitable media for efficient biofilm formation and microorganism immobilization [2]. Therefore, their application in environmental technologies such as biofiltration [3], [4], sorption of pollutants [3], [5], [6], [7] and pesticides [8] has been recently reported.

The first mention of the use of PUR foams as biofilm carriers in wastewater treatment plants (WWTPs) appeared in the 1990s [9], [10]. The application was aimed to prolong the function of activated sludge and to intensify the development of nitrification and denitrification processes. Aerated reactors (activation tanks) containing PUR foam carriers have exhibited increased microorganism concentrations in the activated sludge and consequently intensified nutrient removal (substances containing carbon, nitrogen and phosphorus) which helped to reduce eutrophication; lower concentrations of nutrients leave WWTP with less burden on the environment [11], [12], [13], [14], [15]. Moreover, PUR foams have also been shown to possess good sorption properties for organic pollutants in wastewater (residual pharmaceuticals, persistent organic pollutants etc.) [16], [17]. In addition, the study of Chu et al. [13], confirmed higher genus/species diversity of microorganisms in reactors with PUR foam carriers than in conventional reactors [13]; typically, in ecotoxicology studies, biodiversity is an important marker, signifying the quality of the studied medium. PUR foams have also been tested in anaerobic reactors for the capture of sulphurous decomposition products where their unpleasant odour is inconvenient for the surroundings of biogas plant stations [18], [19]. The application of PUR foam carriers increased the yield of sludge in anaerobic reactors, thus helping to transform organic waste into methane as well as to reduce the amount of nutrients-especially nitrogen and carbon [20], [21], [22].

Recently, PUR foam carriers have also been studied in domestic WWTPs [23] and shown to decrease both COD_Cr (chemical oxygen demand, corresponding to the amount of carbon substrate) and ammonium-N (NH₄⁺ and NH₃) values in drain water [24].

However, all the above-mentioned studies used conventional aromatic PUR foams as inert carriers for microbial attachment, where the energy source for (micro)organisms (bacteria and fungi) growth was provided from soluble organic impurities in wastewater. In the case of low-load (typically domestic) WWTPs, it is difficult to ensure a constant supply of nutrients which can lead to the death of microorganisms and thus to a significant reduction in the efficiency of biological wastewater treatment. Therefore, biodegradable polymers, such as cross-linked starch/polycaprolactone or poly(3-hydroxybutyrate-co-3-hydroxy valerate)/polylactic acid blends, have been tested in WWTPs serving as (micro)organism carriers and simultaneously as an additional source of nutrients [25], [26]. So far, these biodegradable blends used were prepared as highly dense, bulk materials with low surface area and consequently with relatively low ability to support biofilm growth. Therefore, it is highly desirable to design a highly porous, biodegradable material that can be applied simultaneously as an efficient biofilm carrier and nutrient source for WWTP.

In this work, fully aliphatic polyester-ether PUR foam was prepared and used as a biodegradable biomass carrier and alternative nutrient source for a 1-month laboratory test in a simplified SBR (sequence badge reactor) set-up simulating domestic WWTPs [27]. The period of one month was chosen based on several studies [12], [28], [29] showing that firstly, after this time period the biomass has reached a steady state of a balanced ecosystem and/or secondly, a short-term reaction of the biomass simulating periods of nutrition supply shortage in low-load WWTPs was studied and 1 month would be the maximum time for such a low-load period. The effect of the added biodegradable PUR foam carrier on biofilm development and diversity in the activated sludge was evaluated together with enzymatic activity and hydro-chemical parameters.