Biochemical Engineering Journal 93 (2015) 102–107 Contents lists available at ScienceDirect Biochemical Engineering Journal jo ur nal home p age: www.elsevier.com/locate/ bej Regular article The feasibility of citric acid as external carbon source for biological phosphorus removal in a sequencing batch biofilm reactor (SBBR) Artur Mielcarek a , Joanna Rodziewicz a , Wojciech Janczukowicz a,∗ , Arthur Thornton b a University of Warmia and Mazury in Olsztyn, Department of Environment Engineering, ul. Warszawska 117a, Olsztyn 10-719, Poland b Atkins, Woodcote Grove, Ashley Road, Epsom KT18, United Kingdom a r t i c l e i n f o Article history: Received 18 March 2014 Received in revised form 9 September 2014 Accepted 4 October 2014 Available online 13 October 2014 Keywords: Fixed-bed bioreactors Fed-batch culture Waste-water treatment Phosphate release and uptake rates Citric acid Aeration and mixing phases duration a b s t r a c t The aim of this study was to evaluate the feasibility of citric acid as external carbon source for biolog- ical phosphorus removal. Citric acid was the only source of organic carbon and biomass occurred only as a biofilm. The effect of varying the duration of the mixing and aeration phases on the process of dephosphatation and consumption of organic substrate was examined. The operation consisted of filling (0.5 h), mixing (2.75–8.25 h), aeration (2.75–8.25 h) and draw (0.5 h) phases. Total phosphorus and nitro- gen influent concentrations were 7.02 mg P L -1 and 2.05 mg N L -1 , respectively. Enhanced intracellular accumulation of polyphosphates (poly-P) and their release in reactors with immobilized biomass (SBBR) was stated. The highest rates of phosphates release (0.059 ± 0.004 g P–PO 4 3- m -2 h -1 ) and phosphates uptake (0.028 ± 0.002 g P–PO 4 3- m -2 h -1 ) were observed for mixing to aeration phase duration ratio of 1:3. The percentage of phosphorus in dry matter of biofilm reached 4.6%. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Rotating biological contactors (RBC) as well as activated sludge sequencing batch reactors (SBR) have been designed for organic compounds removal [1]. These types of treatment technology are also increasingly required to be more effective in removing nitrogen and phosphorus in response to the tighter water quality objectives. Expectations regarding increasing quality of wastewaters dis- charged to the natural environment have urged the need for increasing the effectiveness of nitrogen and phosphorus removal in technological solutions of this type. In some studies the clas- sic sequencing batch reactors are shown to be superior to the continuous-flow systems (flexibility in respect of flow and load of contaminants) [2,3], and reactors with biofilm are superior to the suspended activated sludge systems (lesser susceptibility to changes in the load of contaminants, lesser biomass growth, higher biomass concentration, lower volume of reactors) [4–6], sequencing batch biofilm reactors (SBBR) are considered as a novel treatment approach. ∗ Corresponding author. Tel.: +48 89 5233257; fax: +48 89 5234124. E-mail addresses: artur.mielcarek@uwm.edu.pl (A. Mielcarek), joanna.rodziewicz@uwm.edu.pl (J. Rodziewicz), jawoj@uwm.edu.pl (W. Janczukowicz), arthur.thornton@atkinsglobal.com (A. Thornton). Phosphorus removal in wastewater biological treatment sys- tems requires the incorporation of the phosphorus into the biomass and then the removal of the excess biomass. The mean content of phosphorus reaches 2–3% d.m. of bacte- rial cells [7,8]. However, some microorganisms are capable of its increased uptake and intracellular accumulation. The mechanism of enhanced biological phosphorus removal (EBPR) has applied for years been in technologies developed for nutrients removal from wastewater treatment [1]. Traditional heterotrophic denitrification and biological dephos- phatation are, to a large extent, dependent on the quantity and type of biodegradable forms of organic carbon in respect of both total duration of reactor cycle and duration of its particular phases [5,9–12]. With the appropriately duration of the anaerobic phase and the presence of sufficient organic substrates, the dephosphata- tion may be only slightly delayed or even occur simultaneously with denitrification. The problem of the insufficient quantity of the organic compounds may be solved by applying an external source of organic compounds. The impact of different types of organic com- pounds on the effectiveness of nitrogen compounds removal or the capability of activated sludge for enhanced intracellular accumula- tion of polyphosphates has already been extensively investigated [9,13–15]. One of the most popular and the best explored sources of organic carbon in reactors with activated sludge is acetic acid. How- ever, volatile fatty acids (VFA) [16–18], and substrates like glucose, methanol, ethanol [14,19–22] as well as selected types of industrial wastewaters [23,24] or waste products [25–27] may serve as good http://dx.doi.org/10.1016/j.bej.2014.10.001 1369-703X/© 2014 Elsevier B.V. All rights reserved.