This study investigates the effects observed on operational parameters in a large and full-scale wastewater treatment plant subjected to anaerobic reject water (ARW) diversion off the main line for a 3-month period and further monitoring for a 2-year period. The plant's secondary unit consists of a two-stage plug-flow-modified Bardenpho process receiving wastewater from both municipal and industrial origins. As a result, ARW was found to have a direct effect on bulking in secondary clarifiers and foaming in anaerobic digesters (AD) despite its relatively small flow rate. During the cut-off period a highly stable sludge volume index at 80 mL g−1 level was obtained in the secondary clarifiers, effluent suspended solids concentration was reduced and continuous feeding to AD was recovered. Sludge density increased in the thickeners during hot season. Secondary clarifiers showed good and stable settleability despite low dissolved oxygen, food/microorganism ratio and high sludge retention time and ammonium levels in the biological unit. The bulking and foaming effect was presented on the plant's internal flow balance. ARW needs serious consideration for elimination by appropriate technologies because of its high potential as a multi-dimensional pollutant source, not only as a carrier of nutrients but also as a possible carrier of filamentous bacteria, which might promote chronic seeding and retention in the system.
Anaerobic reject water (ARW) disturbs several mass balances both on the main and sludge lines reaching N overload at 10–30% despite its relatively low flow rate in wastewater treatment plants (WWTPs) (Vandaele et al. 2000; EPA 2007). Side-stream treatment applications enable reductions in nitrogen load and sludge production, and savings in aeration on the main line with higher effluent quality for nitrogen (Carrio et al. 2003; Joss et al. 2009). Other effects of ARW in WWTP operational parameters can be more profound due to the transportation of filamentous bacteria (FB) responsible for foam forming and bulking to the main line as ARW can contain some solids after decantation (Pitt & Jenkins 1990; Pagilla et al. 1998). Internal recirculation of ARW to the main line is a possible route for chronic seeding of FB in the activated sludge unit. FB-related foaming in the activated sludge also affects anaerobic digesters (AD) resulting in altered sludge feeding.
In the present study, the aim was to investigate the effects of ARW on settleability and related operational parameters in a large and full-scale WWTP, which experienced excessive foaming in the aerated zones of its Bardenpho Unit (BU) and AD. Aeration and sludge feeding were altered. A more severe problem arose as the sludge that could not be fed to the digesters was carried via the supernatant of the thickeners to the inlet of the plant creating a vicious circle of solids between the primary settlers and sludge thickeners. Added solid load on the primary settlers contributed to the solid rise in the BU during peak flow hours lowering the food to microorganism (F/M) ratio. Primary sludge concentration reached levels well over the design value. As solids consisted mostly of volatile structures, another source of pollution was created as they continued to degrade under anaerobic conditions on the sludge line.
In order to sort out the ARW role in several operational disturbances, ARW was cut off the main line flow in Konya WWTP and changes in the operational parameters were examined on settling, solids, N load, thickening, oxygen levels and internal flows including sludge feeding of the digesters on a year (2011) scale. Supporting data from the following year (2012) were also supplied.
MATERIALS AND METHODS
The WWTP located in Konya City (Turkey) is a large-scale treatment plant receiving both industrial and domestic wastewater at a flow rate of 160,000 m3 d−1 with a million population equivalence in operation since September 2010. The plant is operated by Konya Water and Sewerage Administration (KOSKI) and consists of primary (grit removal and primary sedimentation) and secondary treatment (modified Bardenpho) units for organic matter and partial nitrogen removal. BU (77,000 m3) consists of two-parallel plug-flow lines with four oxic zones with 56,980 m3 and anoxic zones of 20,020 m3 volumes. Four 7,000 m3 AD receive combined and thickened primary and waste activated sludge (WAS) and are operated with a hydraulic/sludge retention time (HRT = SRT) of 20 days.
Design values for mixed liquor suspended solids (MLSS) in BU inlet zone, F/M ratio, thickened sludge solid content and BU SRT were 3,800 mg L−1 MLSS, 0.19 kg BOD5/kg MLSS d−1, 5% and 9.4–11.2 d, respectively, while obtained levels were 4,200–5,300 mg L−1, 0.11–0.13 d−1, 2.6–2.7% and 17–23 d, respectively, in the beginning of 2011. A lowered F/M ratio and higher SRT were due to excessive suspended solids (SS) concentration in BU. Foaming in the aerated zones of BU and bulking were the operational problems encountered on the main line. On the sludge line, low density of the thickened sludge and foaming in AD caused solid accumulation in the plant. The studied period of the plant was the year of 2011 for the cut-off effect and 2012 for normal plant operation.
ARW is produced as the digester supernatant from the decanter at a flow rate of 1,400 m3 d−1 and is recycled to the inlet of the WWTP. Its characteristics were monitored for a period of 8 months from May to December 2011 and summarized as 335–1,380 mg L−1 for chemical oxygen demand, 150–900 mg L−1 for biochemical oxygen demand (BOD5), 607–1,120 mg N L−1 for total nitrogen, 580–900 mg L−1 for NH4+-N and 21–102 mg L−1 for total phosphorus. It had a considerable level of BOD5 in addition to N and P with around a BOD5/N ratio of 1.1.
ARW was cut off the plant inlet for a period of 3 months between June and August 2011. The study was conducted with routine monitoring of the parameters as SS, volatile solids, MLSS, sludge volume index (SVI), dissolved oxygen (DO) and total solids. Ammonium nitrogen (AN) was monitored in the influent wastewater, primary settler effluent (PSE) and final effluent. All parameters were analyzed according to Standard Methods (APHA 2005). DO concentration was monitored via online oxygen meters.
RESULTS AND DISCUSSION
Description of the operational problems
In KOSKI WWTP, excessive biomass concentration in the BU was accompanied by bulking in the secondary clarifiers. Temporary foam production resulted in bubbling over the aerated zones of the BU and was controlled by altering the aeration. Defoam agents were applied periodically with only temporary effect. Severe bulking also increased effluent BOD and SS concentrations to violation level. Foam production also took place in the AD and sludge feed was shut off automatically, resulting in sludge overflow from thickeners, which was also returned to the main line via internal recirculation.
To investigate the cause of bulking and foaming, FB species were examined and Types 0041/0675, 021N, 0092, 1851, 0581, 0803, N. Limicola I, II, III, M. Parvicella, Zooglea and Thiothrix II were found as dominating filamentous species in the plant during the winter months, whereas N. Limicola II, III, Types, 021N, 0092, 0041/0675, 0581, 0803, M. Parvicella and Nocardia dominated in the hot season (Aygun et al. 2013; Yel et al. 2013). Among the foam forming filamentous species, Nocardia amarae, Nocardia pinensis, Microthrix parvicella (Goddarao & Forster 1987; Blackall et al. 1989; Pitt & Jenkins 1990), M. parvicella stands out as the dominant species in both modified Bardenpho (BNR) and Phostrip plants related to foaming and bulking with an F/M ratio lower than 0.2 (a favorable condition for M. parvicella) (Baxter-Plant et al. 1999) and AD receiving fatty matter (Lienen et al. 2014). As many species were present in the studied BU, a cut-off strategy was implemented to sort out the effect of ARW on FB-related operational problems.
Effect on SVI, SS, MLSS and flow rates
During the ARW cut-off period, SVI values were significantly reduced in BU, dropping down to below 100 mL g−1 level and stabilizing between 60 and 80 mL g−1 ending a severe bulking period (Figure 1(a)). After that period, BU SVI gradually increased to 150–200 mL g−1 level returning back to bulking condition. To compare the plant's normal SVI levels in the summer months, SVI data of the year 2012 are provided in Figure 2(b). An earlier decrease in SVI started in the spring months at a fluctuating pattern between 80 and 120 mL g−1 and ended in August. Temperature ranged at a similar level during both periods; thus, it was not an influential factor on the results. Elimination of ARW seemed highly efficient in establishing a highly stable and low SVI at 80 mL g−1 where its addition bounced it to the 120–130 mL g−1 level, indicating a net increase. Data observed in the 2012 summer period started to increase to 130–170 mL g−1 level much earlier (in August) despite the high temperature environment. Further rises in SVI with lowering temperature in the following months might be correlated to M. parvicella dominance as reported more frequently in winter and spring than in summer and autumn (Kristensen et al. 1994; Eikelboom et al. 1998; Kruit et al. 2002). Based on the operational experience in KOSKI WWTP, the temperature difference between day and night exceeding 10 °C acts as a major factor on the dominance of FB as floc-forming bacteria are not tolerant to such conditions and lose their competing ability.
With the start of the ARW cut-off period, there was limited success on the controlling and reduction of MLSS build up in BU (Figure 2(a)). Elevated levels of solids transported to the plant due to heavy rains in May had exceeded the settling capacity of the primary settlers and increased MLSS level in the BU; therefore, the nutrient effect of ARW on biomass growth could not be determined. Nevertheless, return sludge concentration reached a level above 15,000 mg/L as a result of higher settleability obtained in the secondary clarifiers. This phenomenon is crucial in the controlling of BU SRT and increasing sludge density in the thickeners. The unstable and fluctuating pattern of the return sludge concentration at 5,000–15,000 mg/L in 2012 indicated a continuous disturbance in the settling ability (Figure 2(b)).
Improvement in SVI and return sludge concentration was followed by an increasing pattern in the sludge thickening degree and lowering the effluent SS concentration of the WWTP in the summer of 2011 as shown in Table 1. Nevertheless, thickened sludge density showed an opposite pattern compared to the same period under normal operation in 2012. Its density tended to decrease during the summer months despite the same SVI level of the activated sludge and the fluctuating pattern of return sludge from April to August (2012). Observed data supported the fact that temperature was influential in the thickening performance and showed that the cut-off strategy improved thickening despite the temperature effect in the summer of 2011.
|.||Thickened sludge density (2011) (%) .||Thickened sludge density (2012) (%) .||Effluent SS concentration (2011) (mg L−1) .|
|.||Thickened sludge density (2011) (%) .||Thickened sludge density (2012) (%) .||Effluent SS concentration (2011) (mg L−1) .|
Elimination of ARW controlled FB and foaming substantially as evidenced by Erdirencelebi & Kucukhemek (2013). Reduction in high SVI and foaming regulated internal flows of primary sludge, WAS, thickener overflow and sludge feeding closer to the design values as illustrated in Figure 3. Excessive overflow from sludge thickeners occurring in May can be mainly attributed to the higher inlet flow rate of the WWTP during this month, when a higher ratio of return sludge flow was directed to the thickeners to reduce SRT and MLSS in BU. WAS flow rate had a design value of 2,936 m3 d−1 and fluctuated at a large scale around this value until the cut-off period. The maximum hydraulic capacity of the plant (200,000 m3 d−1) was not exceeded, but thickener overflow occurred at levels reaching 9,000 m3 d−1 exceeding considerably its design value of 3,600–4,000 m3 d−1. The primary sludge flow pattern showed that the drop in the AD feeding also overloaded the primary settlers hydraulically. During the foaming in AD, primary sludge had to be drawn at doubling flow rates. It was possible to reduce it to the design value of 1,942 m3 d−1 in the second half of September. Foaming in the AD could be lowered gradually and sludge feeding was increased over 1,000 m3 d−1 during the cut-off period. With the start of ARW recirculation, sludge feeding was reduced again during September. It recovered and proceeded at its design value during the rest of the year.
Results show that bulking and foaming can unbalance and/or deteriorate a WWTP operation, both in quality and hydraulics. Design characteristics are usually based on normal operation with peak flow consideration, but biological processes can always produce unpredictable conditions that need to be taken into consideration during the design stage.
Overloading of AN
AN was monitored in the PSE to determine the overload of recirculated AN during the winter months (Figure 4). All the internal flows were close to their design values during the sampling period. Additional nitrogen load was calculated from the difference between the influent wastewater and PSE ammonium concentrations. It averaged 23% (6–47%) of the influent AN making a load at 2,227 kg N d−1. ARW transported one-third of the extra load where two-thirds were produced from degradation of solids escaping anaerobic digestion.
The actual performance (22–61%) of AN removal was higher than the value calculated (9–51%) between the influent and effluent of the WWTP due to the combined flow and load. (The actual performance went unnoticed). As a result, BU capacity of ammonium oxidation was exceeded considerably as evidenced also by low DO levels in the oxic zones. The capacity was limited during the foaming periods due to altered aeration; therefore, ammonium removal proceeded at a wide range and persisted in the WWTP final effluent at high levels. In ideal sludge feeding, AN overload would have originated only from ARW, which would have returned the organic nitrogen removed in the primary sludge and WAS in the AN form to the main line.
Low DO levels and filamentous growth
Low DO conditions in the activated sludge unit are reported as a promoter of filamentous growth and decreased settleability (Wilen & Balmer 1999). DO profiles in Line-1 BU oxic zones are presented for 2011 and 2012 in Figure 5(a) and 5(b). DO concentration proceeded at low levels during the cut-off period at 0.1, 1.0–2.0, 0.5–2.0 and 0.1–0.8 mg/L in oxic zones 1, 2, 3 and 4, respectively, in 2011. Data for the same period in 2012 present a higher oxygenation level compared with the previous year, especially in oxic zones 2 and 3. This indicates that low SVI values can be obtained at 60–80 mL g−1 despite low DO levels. This supports the theory that the plug-flow regime of BU is advantageous in the elimination of filamentous dominance despite low DO F/M and high ammonium levels (Donaldson 1932; Park & Noguera 2004; Guo et al. 2010). At this point, ARW's role seemed to be a carrier of FB in Konya WWTP. Aerobic, anoxic, anaerobic zone grower FB as M. parvicella, Type 0092, Type 0041/0675 and some species (morphotypes, Type 0961, Type 1863, Type 1851 and N. limicola) have been shown to have the capacity to for fermentative metabolism, finding competitive advantages in environments with anaerobic stages or conditions (Horan et al. 1988; Nowak & Brown 1990; Martins et al. 2004). Anaerobic growth of M. parvicella was reported with RNA sequencing in full-scale biogas plants (Lienen et al. 2014).
The most frequently identified FB in 964 WWTP in France were Type 0092 followed by M. parvicella and Thiothrix sp., mainly related to the sludge treatment process (Graveleau et al. 2005). Lack of sludge evacuation management was found responsible for high MLSS concentrations; thus, reduced oxygen levels in the aeration tank lead to favorable conditions for FB. Excessive levels of G. amarae and M. parvicella in anaerobic digestion were found to cause foaming problems not only in the anaerobic digestion process itself but in a preceding activated sludge system due to their presence in the recycle stream from the dewatering facilities (Pitt & Jenkins 1990; Pagilla et al. 1998).
Elimination of ARW in a full and large-scale WWTP with a modified Bardenpho process had a direct and rapid effect on the reduction of bulking and SVI improving the settling ability in the secondary clarifiers and effluent SS quality. It was shown that low SVI levels can be obtained even at low DO F/M and high ammonium levels. High SVI was found to correlate to lower sludge thickening. AN load added on the main line via internal recirculation averaged 23% of the influent AN, which persisted at high levels in the effluent.
Foaming in the activated sludge has been a pre-alarm for AD operation as parallel foaming occurred and sludge feeding was disrupted. Improvement was obtained gradually after the ARW cut-off. Regarding the experience and research on biological treatment, bulking and foaming phenomena can be a severe burden on WWTP operation. ARW treatment toward its nutrient load and possible FB content needs further consideration. The Bardenpho process suffered from filamentous growth despite its favorable environment for floc-forming bacteria. More serious disturbance occurred on the sludge line when AD operation was disrupted. Efficiency of the sludge thickening process has an eminent role on the recirculation flow pollutant load and mechanical systems would be a better choice compared to conventional gravity thickeners to provide lower sensitivity to changes in SVI.