The Influence of a Selected External Carbon Source on the Share of COD Fractions and the Speed of Denitrification Processes

The article presents the effectiveness of the N, P, C (nitrogen, phosphorus and carbon) removal from sewage with the use of molasses as an external carbon source (ECS). The research was carried out during the wastewater treatment processes in two SBR-type activated sludge reactors. A cycle of the SBR operation lasted 360 minutes and included the following phases: wastewater supply (2 min), anaerobic (60 min), aeration (210 min), sedimentation (60 min) and decantation (30 min). After twenty minutes of the wastewater supply, molasses was added to one of the reactors in the cycle, as a source of easily available organic compounds. The conducted tests showed that the use of molasses as an ECS during wastewater treatment resulted in higher nitrogen removal efficiency in comparison with the reactor without ECS. The wastewater treatment in the SBR without the addition of ECS resulted in a total nitrogen removal of 80% and ammonium nitrogen of 95.9%, whereas the wastewater treatment in the reactor with the addition of molasses resulted in the removal of total nitrogen at 90.3% and ammonium nitrogen at 97.3%. Despite the increase in the final nitrate concentration in both SBRs, the nitrate concentration in the reactor using an external carbon source was lower by as much as 4.5 mg N/dm3. The COD fractions and their changes in wastewater were determined in order to find out their quantitative and percentage share. Determination of the COD fraction allows assessing the susceptibility of wastewater to biological treatment, additionally indicating the impurities that are difficult to decompose, which reduce the effectiveness of biological wastewater treatment. Introduction of ECS in the form of molasses to municipal wastewater caused an increase in the SS fraction by 9 mg O2/dm 3, thus increasing the percentage of the readily biodegradable dissolved organic compounds by about 10%. The increased amount of easily available carbon compounds has contributed to the increase of the denitrification rate. In the initial phase of denitrification with the addition of ECS in the form of molasses, an acceleration in the removal of nitrogen compounds by 2.48 mg N∙dm3/h compared to the control reactor, was observed.


INTRODUCTION
The supply of an external source of carbon to the treated wastewater is often necessary to achieve a high efficiency of the wastewater treatment plant, which must meet very strict requirements for reducing the nitrogen concentrations. The use of conventional sources of coal results in high operating costs for wastewater treatment plants. The costs of an external coal source and waste management together account for more than 50% of the total cost of wastewater treatment  Janczukowicz et al. 2011Janczukowicz et al. , 2013. This has become a premise for exploring other, alternative sources of organic carbon.
Molasses is a waste product that has a high potential as an external source of carbon. It is created as a by-product of the sugar industry and often used in the distillery industry. Molasses is a malleable liquid with a brown colour. The substance has a specific smell and a bittersweet taste. Molasses contains about 48-50% sucrose (Arshad et al. 2008, Silva et al. 2009, Smyk and Ignatowicz 2017. Molasses or hydrolysed molasses can be used to support biological wastewater treatment processes. The main component of molasses, i.e. polysaccharides, contains long chains that prevent rapid use of this substrate by denitrifying bacteria; therefore, it is recommended that molasses be hydrolysed to convert it into simpler compounds such as sucrose, fructose and glucose (Janczukowicz and Rodziewicz 2013). The difficulty in using molasses as ECS is its high density, which can create problems with the precise dosing of the substance (Arshad et al. 2008, Mąkinia and Czerwionka 2013, Silva et al. 2009).
The aim of the research was to determine the effect of molasses on the speed and efficiency of the denitrification process as well as the changes in COD fractions during individual wastewater treatment phases.

MATERIALS AND METHODS
The tests were performed during municipal wastewater treatment processes carried out in two independent reactors (R1 and R2) with SBR-type activated sludge (Smyk and Ignatowicz 2017). Their active volume was 13 dm 3 , whereof 8 dm 3 was occupied by activated sludge, and the remaining volume was occupied by mechanically treated raw wastewater delivered from the Białystok wastewater treatment plant. Table 1 shows the operating parameters of the SBR reactors.
A single operating cycle of a reactor lasted 6 hours and comprised the following stages: wastewater supply (2 min.), anaerobic mixing (60 minute), aeration (210 min), sedimentation (60 min) and decantation (30 min). During the stage of aeration performed via a diffuser located at the reactor bottom, compressed air was supplied, depending on the operation stage, in amounts from 0.1 up to 2.0 mg O 2 /dm 3 ; activated sludge concentration was 3.5 kg/m 3 , sludge index fluctuated from 120 to 180 cm 3 /g, chamber hydraulic load was 1.5 m 3 /m 3 •d and the load of organic compounds was 0.2 kg COD/m 3 •d. Molasses were added to one of the SBRs 20 minutes after filling the wastewater. The dose of molasses was individually calculated taking into account the amount and composition of raw wastewater, assuming the COD/N ratio of 6.
The wastewater samples were filtered, and each of the following parameters were determine individually, in accordance with the applicable methodology ( The rate of NUR denitrification was determined using the formula:

RESULTS AND DISCUSSION
The obtained results are shown in Figures 1-5. According to the guidelines of ATV, Henze, Tora (2011), Yang (2012) and other authors, denitrification takes place without any disturbances if the COD/N ratio is between 5 and 10. The COD/N ratio in the wastewater of the SBR reactors tested was insufficient for the denitrification process and was lower than the recommended range. The COD/N ratio in the raw wastewater was 7.2, while the reactors were filled with wastewater and the 20-minute mixing process reduced this value to 4.2. Adding molasses to the reactor increased the COD/N ratio to the required value ( Figure 1).
The amount of organic compounds defined as COD in the mechanically treated wastewater was 535 mgO 2 /dm 3 and BOD 5 300 mgO 2 /dm 3 ( Figure 2). The concentration of N tot in raw wastewater was 73.9 mgN/dm 3 , ammonium nitrogen -51.1 mgN/dm 3 and nitrates -2.6 mgN/dm 3 ( Figure 3 decrease in the amount of organic matter was noted 20 minutes after the SBRs were filled, which indicates that the assimilable carbon compounds were absorbed by the denitrifying bacteria. The amount of organic compounds measured as COD in the control reactor was 268 mgO 2 /dm 3 and BOD 5 130 mgO 2 /dm 3 . Similar values were recorded in the second R2 reactor: COD was 259 mgO 2 /dm 3 and BOD 5 130 mgO 2 /dm 3 . The concentration of ammonium nitrogen in R1 was 44.9 mgN/dm 3 and 45.8 mgN/dm 3 in R2. The concentration of nitrates increased slightly, in SBR R1 the concentration was 3.9 mgN/dm 3 , whereas in SBR R2 -4.0 mgN/dm 3 . Molasses were then added to SBR R2 and after twenty minutes of anaerobic denitrification, further samples were taken. In the control SBR, where the wastewater treatment process was carried out without the assistance of ECS, a further slight decrease in the amount of organic compounds defined as COD (262 mgO 2 /dm 3 ) and BOD 5 (120 mgO 2 /dm 3 ) was observed (Figure 2). The introduction of ECS in the form of molasses caused an increase in the COD values in SBR R2 to 281 mgO 2 /dm 3 . The concentration of N tot in SBR R1 decreased to 55.9 mgN/dm 3 (Figure 3). InSBR R2, the concentration of N tot also decreased to 54.0 mgN/dm 3 . In the case of nitrates, a lower concentration was recorded in R1 than in the SBR with added molasses -the concentration in R1 was 2.4 mgN/dm 3 , while the concentration in R2 was 2.9 mgN/dm 3 . The concentration of ammonium nitrogen decreased slightly in the reactor R1 to 43.2 mgN/dm 3 . In SBR R2, a high decrease in ammonium nitrogen concentration to 41.0 mgN/dm 3 was recorded.
After another twenty minutes of anaerobic wastewater treatment, a further decrease in COD (259 mgO 2 /dm 3 ) was observed in the R1 and BOD 5 reactors (115 mgO 2 /dm 3 ). COD was also reduced to 273 mgO 2 /dm 3 and BOD 5 to 115 mgO 2 /dm 3 in the reactor with molasses ( Figure 2). A clear difference in nitrogen form concentrations between the reactors was observed. The concentration of N tot continued to decrease in SBR R1 and was 53.6 mgN/dm 3 . In the case of SBR R2, the concentration of N tot was 43.3 mgN/dm 3 (Figure 3).
The COD value in the wastewater treated from the R1 reactor after decantation was 37 mgO 2 /dm 3 , while BOD 5 was 9 mgO 2 /dm 3 (Figure 2). Despite the addition of ECS in the wastewater treated with molasses, COD was 25 mgO 2 /dm 3 and BOD 5 4.0 mgO 2 /dm 3 . These values are lower than in SBR R1, where no additional ECS was used. The concentration of N tot in the wastewater treated from SBR R1 was 14.8 mgN/dm 3 , ammonium nitrogen -2.1 mgN/dm 3 and nitrates -8.8 mgN/dm 3 (Figure 3). In SBR R1, the removal of N tot took place with an efficiency of 80% and ammonium nitrogen -of 95.9%. In the case of SBR R2, the concentration of N tot in treated  In SBR R2, the removal of N tot occurred with an efficiency of 90.3% and ammoniacal nitrogen of 97.3%. The use of molasses resulted in a higher efficiency of the wastewater treatment process and, above all, in a more effective removal of nitrogen compounds. During the study, the COD fractions in the wastewater were determined in order to determine their quantitative and percentage share. Determination of the COD fraction allows the assessment of the susceptibility of wastewater to biological treatment, additionally indicating the pollutants which are difficult to decompose and reduce the effectiveness of biological wastewater treatment. A distinction is made between easily biodegradable, hardly biodegradable and non-biodegradable compounds. The biodegradable fraction includes the fraction of dissolved organic substances easily biodegradable S S and the fraction of insoluble organic substances slowly biodegradable X S . The non-biodegradable fraction includes fractions of dissolved non-biodegradable organic substances S I and insoluble non-biodegradable organic substances X I ( Figure 4 shows the quantitative share of the individual fractions in individual reactors. The fraction of insoluble, slowly biodegradable X S organic compounds dominated in the mechanically treated raw municipal wastewater. This fraction accounted, on average, for 46.6% of the total COD of wastewater. The percentage share of the fraction of dissolved organic compounds easily biodegradable S S in raw municipal wastewater was 33.8% on average. The fraction of insoluble non-biodegradable organic compounds X I accounted for 15.5% of the total COD of wastewater. The fraction of dissolved non-biodegradable organic compounds S I had the lowest percentage share in raw municipal wastewater -4.1%.
After filling the SBRswith wastewater and mixing for twenty minutes, a decrease in the fraction of insoluble organic compounds slowly biodegradable X S was observed by 202.5 mgO 2 /dm 3 on average. A decrease in the fraction of easily biodegradable organic compounds S S was also observed from the mean value of 211.0 mgO 2 /dm 3 to 132 in the control reactor and to 136 mgO 2 /dm 3 in the reactor with molasses. The percentage shares of the S S fractions increased to 49.64% for R1 and 52.20% for R2. The value of the fractions of insoluble non-biodegradable organic compounds X I also decreased, on average, by 67.5 mgO 2 /dm 3 , which in both reactors accounted for about 10% of the total COD of wastewater. The fraction of dissolved non-biodegradable organic compounds S I did not change its value as a result of filling the SBRs, while its percentage share increased slightly and ranged from 5.0 to 7.9%.
Molasses was added to the R2 reactor. After twenty minutes of wastewater mixing in the reactors with added molasses, an increase in the amount of dissolved, easily biodegradable organic compounds determined by the SS fraction was noted at 9.0 mgO 2 /dm 3 . In the control reactor, a decrease in the value of readily biodegradable organic compounds fraction at 3.0 mgO 2 /dm 3 was recorded. In both SBRs, a decrease in the value of X S and X S fractions of insoluble, slowly biodegradable organic compounds and in the value of non-biodegradable, non-biodegradable organic compounds X I fractions was recorded, which is caused by the combination of the X I fraction with activated sludge flocculation.
The increased amount of easily assimilable carbon compounds has contributed to an increase in the denitrification rate. Figure 5 shows the average denitrification rate of NUR. In the case of the denitrification rate in a SBR without addition of ECS, the speed was much lower and amounted to only 0.45 mgN·dm 3 /h, while the difference in rate between the SBRs was 2.48 mgN·dm 3 /h.

The application of molasses as ECS in waste-
water treatment increased the effectiveness of removing the forms of nitrogen while maintaining a high efficiency of removing organic compounds.
2. The use of molasses reduced the nitrate concentration in the treated wastewater by 4.9 mgN/dm 3 compared to the SBR without ECS support.
3. The introduction of molasses into municipal wastewater resulted in an increase of 10% of the dissolved, easily biodegradable organic compounds called SS fraction in the R2 reactor.
4. Molasses, as a waste product, can be successfully used as an external source of carbon in denitrification processes.