VojtÄch PospÃÅ¡il*, Petr Kelbich*, Lucie Chovancová*, FrantiÅ¡ek MartÃnek*, Martin ..... J., ÄECH, J.S., KOS, M. New process design for biological nutrient removal.
Study of regeneration zone in WWTP Liberec Vojtěch Pospíšil*, Petr Kelbich*, Lucie Chovancová*, František Martínek*, Martin Pečenka*, Iveta Růžičková*, Jiří Wanner* *University of Chemistry and Technology, Prague Abstract This paper deals with the study of basic functions of activated sludge regeneration zone, particularly in its application for bioaugmentation of nitrification in situ. Keywords Activated sludge, nitrification, regeneration, bioaugmentation in situ
NITRIFICATION – REGENERATION AND BIOAUGMENTATION IN SITU The main role of modern activated sludge process for wastewater treatment is to remove organic pollution, nitrogen and phosphorus to very low residual concentrations to prevent pollution of receiving waters. The most difficult pollution to deal with is nitrogen and its components present in wastewater. The main process of nitrogen removal is nitrification when ammonia nitrogen is oxidized to nitrate nitrogen which is subsequently reduced by other bacteria to nitrogen gas. This process is performed by chemolithotrophic nitrification bacteria which are slow-growing and very sensitive to external factors. Therefore it is very difficult to maintain enough nitrification bacteria in activated sludge. The bioaugmentation of nitrification in situ is one of possible ways how to enhance the nitrification population in activated sludge. This method is often combined with activated sludge regeneration for bulking control. Both processes bioaugmentation and regeneration are performed in the side-stream of return activated sludge in a separate reaction zone.
Function of the regeneration tank. Regeneration zone has three basic functions and these are (i) regeneration of the storage capacity of cells, (ii) increasing the aerobic sludge age and (iii) nitrification bioaugmentation in situ. Another feature is the accumulation of sludge / biomass. This is used for example in WWTP Opava, where there is a potential toxic inflow from local pharmaceutical industry, where the accumulated sludge from the regeneration can be used to inoculate the aeraton tank. There is a regeneration tank same as the aeration tank. Generally denitrification function is not in regeneration zone, but sometimes it does. There is a potential for denitrification of produced nitrate. For this purpose sometimes branch part of the influent is pumped into anoxic part of regeneration zone, which is for example in WWTP Liberec. Regeneration of storage capacity of the cells is happening by passing regeneration tanks and by consuming storage compounds. This leads to the reduction of the endogenous respiration rate. Higher endogenous respiration rate at the beginning and lower endogenous respiration rate at the end of regeneration tank confirms its function to regenerate. In the RDN (Regeneration, Denitrification, Nitrification) arrangement of WWTP a regeneration tank also has an influence on the formation of floc forming bacteria and thereby serve to improve the sedimentation properties of sludge. Reducing the respiration rate is just a sign that regeneration was successful; the main role of regeneration is thus returning the cells to the main reactor "hungry". Reduction of the concentration of easily biodegradable substrate proceeds as follows. In the first stage easily biodegradable organic substrate is removed in the activation tank. When the concentration of easily biodegradable substrate is nearly constant and the sludge is aerated in the regeneration tank than the additional consumption of storage compounds happens. The advantage of regeneration tank is that the sludge is concentrated already. Another function of the regeneration zone is to increase the aerobic sludge age, which is defined as the weight of biomass in aerated reactors to the total amount of biomass in the system. It should be noted that the very high aerobic sludge age is not sufficient to support the growth of nitrifying bacteria, if there is not enough substrate in the system. This increase is used to create better conditions for slow-growing nitrifying bacteria. Bioaugmentation in situ. Bioaugmentation of nitrification in situ is based on the supply of substrate for nitrifying bacteria. The principle of this method is to pump into the regeneration tanks substrate for nitrifying bacteria in the form of reject water, which serves as a source of ammonia nitrogen. It is important not to confuse this method with "simple" bioaugmentation nitrification, when nitrifying bacteria are dosed into the activation tank directly. The advantage of in situ bioaugmentation nitrification is that nitrifying bacteria are cultivated in system, so they are used to the environment.
Wastewater treatment plant Liberec - Full-scale plant with bioaugmentation in situ Samples for experimental measurements were taken at the wastewater treatment plant in Liberec. This is a mechanical-biological treatment plant with a capacity of 190 000 PE in the city of Liberec and Jablonec nad Nisou. WWTP is organized into six parallel lines containing primary settling tank, anoxic selector, denitrification, nitrification and rectangular secondary settling tank. For each pair of lines there is one assigned regeneration tank. Samples were taken from five sampling points. The first sampling point was an anoxic selector. Wastewater after primary settling flowing from the well was collected here. This sample was used to dilute the kinetic denitrification tests and served as a source of easily biodegradable substrate. The second sampling point was nitrification tank. Activated sludge and foam was collected here. They were used both for FISH analysis and for microscopic tests. The third and fourth sampling point was at the beginning and at the end of the regeneration tank. Activated sludge was collected for nitrification and denitrification kinetic tests and respirometry tests. The fifth sampling point was effluent from the WWTP. Collected water was used for fractionation tests of COD Cr.
RESULTS: The experiments were obtained in the period of December 2013 – April 2014 at WWTP Liberec. The results from 16th April at WWTP Liberec are shown in charts 1.1, 1.2, 1.3; they are showing positive effect of regeneration zone on nitrification rate. This statement is confirmed by table 1.1, where it can be seen higher nitrification rate at the end of the regeneration zone. Charts 1.4, 1.5, 1.6 are showing denitrification in regeneration zone from 4th February at WWTP Liberec. From our previous experiments we have found out that the denitrification rate is the same throughout the regeneration zone, which can be seen from chart 1.4 and table 1.2.
Chart 1.1 N-NO3- concentration during the kinetic test of nitrification with sludge from nitrification zone of regeneration tank WWTP Liberec
Chart 1.2 N-NO2 concentration during the kinetic test of nitrification with sludge from nitrification zone of regeneration tank WWTP Liberec
Chart 1.3 N-NH4+ concentration during the kinetic test of nitrification with sludge from nitrification zone of regeneration tank WWTP Liberec
Chart 1.4 N-NO3 concentration during the kinetic test of denitrification with sludge from denitrification zone of regeneration tank WWTP Liberec
Chart 1.5 N-NO2 concentration during the kinetic Chart 1.6 COD concentration during the kinetic test of test of denitrification with sludge from denitrification denitrification with sludge from denitrification zone of zone of regeneration tank WWTP Liberec regeneration tank WWTP Liberec Table 1.1 Specific rates of nitrification in regeneration tank at WWTP Liberec
Table 1.2 Specific rates of denitrification in regeneration tank at WWTP Liberec
The chart 1.7 shows that in March the regeneration tank fulfilled its function to regenerate the storage capacity of cells. There is a noticeable difference between the higher endogenous respiration rate at the beginning of the regeneration tank and the lower endogenous respiration rate at the end of the regeneration tank. However, after an accident occurred in the supply of reject water into the system, bioaugmentation of nitrification in situ stopped working correctly, because of this there was not sufficiently high concentration of nitrates in the regeneration tank. Denitrification and oxidation of easily biodegradable organic compounds could not take place. Due to constant inflow of branched wastewater after primary settling, a further increase in the concentration of easily biodegradable substances occurred. Under these conditions, it was quite impossible to regenerate the storage capacity of cells, thus regeneration tank did not fulfil this function. This fact is proved by the convergence of endogenous respiration rates. The accident of blowers also contributed to the lack of oxidation of easily biodegradable organic compounds. The chart 1.8 shows that from January to April regeneration tank fulfilled its function bioaugmentation nitrification in situ. This statement is proved by noticeable difference between the nitrification rates. The nitrification rate is lower at the beginning of the regeneration tank and higher at the end of the regeneration tank. However, after the aforementioned accident in dosing the reject water into the system bioaugmentation nitrification in situ completely disappeared. The nitrification rate in the regeneration tank was identical to the nitrification rate in the aeration tank. After dosing reject water into the system again, bioaugmentation nitrification in situ was restored, but the difference between nitrification rates and nitrification rates itself were significantly smaller. The second accident in dosing the reject water into the system again stopped bioaugmentation nitrification in situ. The general trend of the development of specific nitrification rates was reported as rising from December 2013 to 25th February 2014. Then, due to blower’s accident, which was in operation until 1. 4. 2014 and by following reject water pump’s accident, specific nitrification rate decreased. Reject water was not pumped from 14th march 2014 to 5th May 2014; this event affected the last two samplings. It can be assumed that the regeneration tank at this time did not fulfil its function, because it was the overdosed by organic substrate with the wastewater after primary settling and it was also poorly aerated.
Chart 1.7 The course of respiration rate during the time
the
endogenous
Chart 1.8 The course of the nitrification rate expressed as rX;N-NH4+ during the time
At 4th February 2014 denitrification rates in different profiles of a regeneration zone were compared. From the resulting values it is clear that the rate of denitrification in various profiles regeneration zone not differ significantly. Overdose organic substrate and insufficient aeration could lead to increased specific rate of denitrification in 16th April 2014. On dosing period without reject water can be documented hypothesis that even high aerobic sludge age itself is not sufficient to increase the intensity of nitrification, if it is not enough substrate for nitric bacteria in the regeneration zone. From the resulting specific nitrification rates on the beginning and end of the regeneration zone we can conclude that bioaugmentation nitrification in situ was not completely suppressed due to the presence of ammonium ions in the activated sludge. Regeneration function of regeneration zone was studied by specific endogenous respiration rates. Table 1.3 shows that in 16th April 2014 was more than 50% higher specific respiration rate than in 4th February 2014, which means that regeneration function of regeneration tank was decreasing. Dosage of wastewater after the primary settling is not enough, because there is no sufficiently high concentration of nitrates in order to enable the organic compounds oxidation by denitrifying bacteria. Increasing the concentration of nitrate is achieved by dosing the reject water with a high concentration of ammonium ions to the beginning of the regeneration zone and by subsequent oxidation to nitrates by nitrifying bacteria. In order to verify the specific denitrification rates and specific respiration rates, recalculation to oxygen units was made. When specific denitrification rate was multiplied oxygen equivalent of 2.86 and 0.8. Coefficient 0.8 is there due to the fact that 80% of the microorganisms are capable to respire under anoxic conditions. Table 1.3 Comparison recalculated specific denitrification rate r Date of collection February 4, 2014 February 25, 2014 rX;red;N-NO36.46 12.89 rex 6.93 13.85
specific exogenous respiratory rate R ex April 16, 2014 Oxygen unit 22.96 mg / (h.g) 22.65
X, red; N-NO3
Chart 1.9 shows the progress of sedimentation properties of activated sludge expressed by sludge index, which were in the optimum range between 80 to 100 ml / g. Stable sedimentation properties of activated sludge wastewater treatment plant Liberec can be related to the presence of a regeneration zone in the activation system. Chart 1.10 shows the increase in the concentration of total nitrogen in the influent, which was accompanied by a trend of increasing CODCr, this prevents the degradation of ratio CODCr / N tot. Chart 1.11 shows the mentioned ratios CODCr / N tot. In 2014 was smaller inflow to the WWTP Liberec, but the amount of pollution was the same. From this follows that the ratio CODCr / N tot is relatively stable. This situation is good for the dosage of external substrate.
Chart 1.9 Dependence of the sludge index in the nitrifying tank at the time
Chart 1.10 Increase in the concentration of total nitrogen
Chart 1.11 The ratio of CODCr to the concentration of total nitrogen in the influent for screens at the time
FISH analysis was performed, which showed that the representation of AOB and NOB bacteria in the profile regeneration tanks was similar. AOB occurred in small and medium-sized compact clusters and their amounts after passing the regeneration zone did not increase. The increased amount of NOB is noticeable, but not sufficiently conclusive. Results are influenced by the fact that samples were taken at the time when the flow of reject water into the regeneration was not working for one month. Results are in table 1.4. Table 1.4 Analysis of AOB and NOB at the beginning and at the end of the regeneration tank.. AOB NOB [%] [%] Beginning 6,0 2,7 End 5,6 2,9 AOB occurred in small and medium-sized compact cluster. High amount of free bacteria were detected (level 3). NOB occurred in small and medium-sized compact cluster, free bacteria were not detected.
Figure 1.1 AOB (Cy3, red/light grey) and total biomass (DAPI, blue/dark grey), the beginning of the regeneration tank, 320x.
Figure 1.2 AOB (Cy3, red/light grey) and total biomass (DAPI, blue/dark grey), the end of the regeneration tank, 320x.
Figure 1.3 NOB (Cy3, red/light grey) and total biomass (DAPI, blue/dark grey), the beginning of the regeneration tank, 320x.
Figure 1.4 NOB (Cy3, red/light grey) and total biomass (DAPI, blue/dark grey), the end of the regeneration tank, 320x.
Microscopic analysis showed that the regeneration tank does not affect the growth of foaming filamentous microorganisms, while the number of filamentous organisms causing classic sludge bulking decreased. This results in improved stable sedimentation properties of the sludge. Microscopic analysis confirmed the presence of relatively well developed flocs that are predominantly solid and compact and 80% are about the size of 150 microns. The microscopic analysis also shows that intermittent dosing of iron and aluminum ions has a positive effect against excessive growth of Microthrix parvicella, so the observed foam at WWTP Liberec is rather formed by nocardioform actinomycetes type GALO. Microscopic analysis revealed the presence of relatively welldeveloped clusters of poly-P bacteria. Although the technological line does not have real anaerobic zone, the mechanism of biological phosphorus removal can occur thanks to anoxic selector, in which there is a significant decrease in redox potential. The reason for low redox potential is rapid consumption of the nitrates, which happens due to dosage of wastewater and returned sludge, as a result of this the system switches to anaerobic.
Figure 1.5 Liberec BP 210114-GS750-character-BP-GALO
Figure 1.6 Liberec 210114-NAT125 character AS + Rotifer
Figure 1.7 Liberec BP 210114-GS750-character-BP-GALO II
Figure 1.8 Liberec 210114-NS1250- cluster poly-P
Figure 1.9 Liberec 210114-NAT125 character AS
Figure 1.10 Liberec 210114-NAT125 character AS II
Figure 1.11 Liberec 210114-GS750- filamentous bacteria
Figure 1.12 Liberec 210114-GS750- filamentous bacteria II
Fractionation of CODCr was performed for wastewater after primary settling and on effluent by physicochemical characterization method according Mamais et al 1993. The results are summarized in table 1.4. Although the wastewater contains a high concentration of easily biodegradable substrate SS, but in overall it is not too concentrated. For this reason it is necessary intermittent dosing of external substrate.. Table 1.4 Fractionation of CODCr in wastewater after primary settling and in effluent April 2, 2014 CODCr Wastewater unfiltered
210
Wastewater coagulated
90
SS
41
Effluent Effluent coagulated
58 49 CODCr
SI XS +XI
49 120
Wastewater unfiltered
156
Wastewater coagulated
55
SS
25
Effluent
37
SI
30
Effluent coagulated
30
XS +XI
101
April 16, 2014
mg/l
mg/l
CONCLUSION: From these measurements we can formulate the following main conclusions. Additional function of denitrification in the regeneration tank was confirmed. Furthermore, it was confirmed by respirometric measurements was confirmed that if there is no sufficient nitrification in regeneration tank, the branched supply of wastewater after primary settling to the regeneration tank is harmful. The accumulation storage capacity is deteriorating. The actual effect of bioaugmentation nitrification in situ was directly confirmed by lower nitrification rate in the beginning and by higher nitrification rate at the end of the regeneration tank. It was indirectly confirmed by approximating these nitrification speeds after accident in dosing reject water. It is important to note that there must not be long-term disruptions in dosing the reject water or other sources of ammonia nitrogen into regeneration tank.
In the initial stages of our laboratory experiments, kinetic tests managed to trace positive effect of the regeneration zone to the level of nitrification. The gradual increase of nitrification speed over time occurred and the positive difference between nitrification rate at the beginning and end of the regeneration zone was observed. This positive development was interrupted by prolonged discontinuation of dosing reject water into the regeneration tank. An important finding was that even after discontinuation of dosing the reject water, the rapid collapse of nitrification did not happen, because nitrification bacteria could also use ammonia nitrogen present in lower concentration in activated sludge. The dosing period without reject water can document the validity of the hypothesis that the high aerobic sludge age is not sufficient to provide stable nitrification unless there is not enough substrate for nitrification bacteria. Continuous dosing of wastewater after primary settling to the regeneration tank to promote denitrification proved to be unsuitable, because the organic substrate was not completely used in anoxic part of regeneration tank due to lack of nitrates. A low concentration of nitrates was due to insufficient nitrification. As a result the gradual slow increase of endogenous respiration rate during the experiments occurred. Despite the increase in endogenous respiration rates, regeneration zone still showed a positive effect on the sedimentation properties of activated sludge, which was composed of well-formed flocs and number of filamentous organisms causing bulking, has been reduced. The observed increased incidence of certain foaming organisms is unrelated to the function of regeneration zone because their presence in the activated sludge is determined by characteristics which the regeneration zone cannot influence.
Acknowledgment: This paper was supported by the contract for SčVK a.s., as contract no. 217 61 31 09. The financial support of the company is therefore highly appreciated. Financial support from specific university research (MSMT No 20/2015)
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