BioWin Modeling and Simulation Report

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CIE4703 Water Treatment

BioWin Modeling and Simulation Report

Group 7

January 19, 2020

Name(s):

Agung Kusumawardhana (5005647) Azzahra Safira Suryanto (5008360)

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1 Description of the case

1.1 Introduction

A sewage treatment plant using bioreactors is used to treat sewage water in this case. This sewage treatment plant is targeted to treat wastewater to reach the effluent target of total nitrogen (TN)

< 10 mg/L -N, ammonia (NH4) < 1 mg/L- N, and COD < 50 mg/L. In the base case, the treatment plant does not have any side stream N-removal. However, side stream N-removal is needed to further treat the reject water from sludge dewatering process, which contains lots of N and can be beneficial to recover nutrient in the bioreactor. For this particular side-stream N-removal implementation, Nitritation - Deammonification configuration will be used.

1.2 Influent characteristics

The influent for this treatment plant is sewage water with dynamic COD and high concentration of total nitrogen (TKN). The raw water influent characteristic is shown in Table 1.

Table 1 Raw water influent characteristics

Parameter Value Parameter Value

Flow (m3/d) 24,000 TKN (mg/L) 45

Average COD (mg/L) 500 PO4-P (mg/L) 4

TSS (mg/L) 269 Alkalinity (mmol/L) 6

VSS (mg/L) 224 pH 7.3

NH4-N (mg/L) 29.7

1.3 Treatment plant units

The treatment plant configuration for the base case consists of primary clarifier, three (3) aerobic bioreactor units, three (3) anaerobic reactor units, secondary clarifier, sludge digester, and filter press.

Figure 1 Treatment plant configuration for base case

1.4 Reject water treatment

Reject water is the water coming from sludge dewatering process, consists of high N concentration, and will be recirculated back into the bioreactors. This water, if not treated, can significantly increase the nutrient loading in the bioreactor inlet. Thus, making the effluent water does not comply with the effluent targets. To treat reject water, Nitrification – Deammonification process will be applied. This process consists of nitrifying bacteria repression to reduce nitrite to N2 without oxidizes it to nitrate beforehand. Bacteria used in this process is called Annamox.

2 Problems and objectives

A wastewater treatment plant with base case configuration is used to treat wastewater with dynamic COD. In the base case configuration, reject water from anaerobic digestion that contain nitrogen in high concentration is returned to the treatment plant to be treated. With this base case configuration, the target effluent quality of COD, Total-N and ammonia was not met.

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4 Target effluent should be met, which can be done through optimizing the operational condition of the base case configuration. After the optimization of the base case is done, it is found that it still does not meet the target effluent. Therefore, treatment of reject water is needed to reduce the concentration of nitrogen from the reject water in order to achieve the target effluent.

3 Optimization of the base case

There is a certain effluent standard that need to be achieved through optimizing the operational flows of the base case. The alteration is based on the mechanism of biological nitrogen removal.

Table 1 shows the alteration that has been made to the base case. The treatment plant configuration for base case can be seen in Figure 1.

Table 2 Base Case Optimization

Number Unit Initial Condition Optimization Reason

1 WAS

splitter

Side stream flowrate 700 m³/day

Sidestream flowrate decreased to 400 m3/day

Nitrifiers bacteria require longer time to grow than heterotrophic bacteria. Reducing the waste activated sludge will increase the SRT and leads to better performance of nitrogen removal.

2 Reactor

#6

DO setpoint at 2 mg/l

DO setpoint decreased to 1 mg/l

Part of wastewater from reactor #6 will be returned to reactor #1. If too much DO is present, denitrification process will be hindered.

3 Internal

return

Rate in side-flowrate 50.000 m³/day

IR 1

Initial condition of base case before optimization was simulated through dynamic simulation for 10 days.The results show that the effluent quality of Total-N and Ammonia has not met the target

effluent quality. After optimization, there are improvements in the total-N and ammonia effluent quality, although the total-N still has not met the target of 10 mg/L. The effluent quality of the

initial condition and after optimization is shown in

Table 3. Figure 2 shows the effluent concentration graph for 10 days simulation after optimization.

Figure 2 Effluent concentration after base case optimization Table 3 Effluent Quality after Base Case Optimization Parameter Initial average

effluent (mg/L)

Initial max.

effluent (mg/L)

Average effluent quality (mg/L)

Max. effluent quality (mg/L)

Targets (mg/L)

COD 41.33 45 44.97 48.69 < 50

Total N 16.63 18 11.95 13.99 < 10

Ammonia 7.98 9.27 0.60 0.90 < 1

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4 Implementation and optimization of the side stream treatment

Nitritation-deammonification treatment train was used for the removal of nitrogen from the reject water from anaerobic digester. From the digester, the wastewater flow will be divided into nitritation reactor and deammonification reactor. Ammonium that flows through nitritation reactor will be converted into nitrite. The nitrite from nitritation reactor is then get into deammonification and combined with ammonium that flows from the splitter. From deammonification reactor, wastewater flows to clarifier, and the sludge is returned to deammonification reactor. After that, the effluent flows to mainstream reactor. The configuration of the sidestream treatment can be seen in Figure 3.

Figure 3 Treatment Plant configuration with side-stream treatment Table 4 Sidestream treatment implementation and optimization

Number Unit Optimization Reason

1 Splitter Ratio S/M 0.5 Ammonium is needed for anammox reaction;

therefore, part of the flows is splitted to flow to deammonification reactor.

2 Nitritation Reactor Volume 270 m³ Partial nitritation needs longer HRT since ammonium oxidizer bacteria is slow to grow. This volume results in HRT of 36.4 hours or 1.5 days.

DO constant at 1 mg/l Ammonium oxidizer bacteria is working in aerobic condition. The DO was set to low level to inhibit the growth of nitrite oxidizer.

Temperature constant at 33^oC

Nitrite oxidizing bacteria are sensitive to temperature, by setting the temperature around 30 – 35^oC, the growth of NOB can be inhibited.

3 Alkalinity Flow 50 m³/day Cation strong base 120 meq/L

Nitritation releases 2 moles of proton for 1 mole of ammonia that is oxidized. These protons will reduce the pH in nitritation reactor. Alkalinity is needed to buffer the pH.

4 Deammonification

Reactor

Volume 300 m³ This volume results in HRT of 16.7 hours.

Temperature constant at 35^oC (Unaerated)

Anammox bacteria grows optimal at temperature around 30 – 40 ^oC in anaerobic condition

5 Clarifier Area 200 m²

Depth 4 m

Underflow 200 m³/day

Return sludge is needed to maintain the SRT of deammonification reactor at around 11 days because of the doubling time of anammox bacteria. By setting the underflow to 200 m3/day, SRT of 10.57 days was achieved.

Table 5 Effluent quality of mainstream treatment with side-stream treatment

Parameter Average effluent quality (mg/l) Max effluent quality (mg/l) Targets (mg/L)

COD 45.05 48.76 < 50

Total N 10.05 11.14 < 10

Ammonia 0.62 0.95 < 1

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5 Discussion

5.1 Sidestream treatment compared to base case

The main point in side-stream treatment is in the presence of anammox process. Partial nitrification is needed in order to get nitrite that will be used as electron acceptor. In nitritation reactor, ammonium is converted to nitrite by ammonium oxidizing bacteria. The key reaction of nitritation is given in equation (van Hulle, et al., 2010):

𝑁𝐻₄⁺+³

2𝑂₂→ 𝑁𝑂₂⁻+ 𝐻₂𝑂 + 2𝐻⁺ (1)

After optimization of side-stream treatment, it is found that anammox bacteria is present in the deammonification reactor with concentration of 650.80 mgCOD/L. Ammonia and nitrite oxidizing bacteria are also present with concentration of 239.76 mgCOD/L and 4.07 mgCOD/L respectively. With the presence of anammox bacteria in high concentration, anammox process was achieved. Table 6 shows the comparison of performance before and after side-stream treatment is implemented.

Table 6 Comparison of maximum effluent concentration between initial base case, after optimization, and after side-stream treatment application

Treatment COD (mg/L) Total N (mg/L) NH4 (mg/L) Power demand (kW) Cost ($/hr)

Initial 45 18 9.27 - -

Base case opt. 48.69 13.99 0.90 181.6 422.76

With Side-stream 48.76 11.14 0.95 181.2 421.98

From Table 6, it can be seen that with the implementation of side-stream treatment to decrease the concentration of total N that flows back into the main treatment bioreactor leads to further reduction of total N in the effluent. Application of deammonification process can also cut the oxygen consumption to do the conventional nitrification process, and thus reducing the power demand for aeration process and cost for power consumption. However, it should be noted that the side-stream treatment is optimized without further optimizing the main configuration (base case). Therefore, to further improved the treatment plant, one final optimization is done to the system.

5.2 Final optimization

After tweaking the sidestream treatment and no further improvement can be seen in the effluent quality, another alteration was made in the main treatment plant summarized in table 7.

Table 7 Final optimization

Number Unit Initial Condition Optimization 1 Reactor #6 DO setpoint at 1 mg/l DO setpoint increased

to 1.5 mg/l

2 Internal recycle IR at 1 IR increased to 2

Dissolved oxygen in reactor #6 was set higher in order to increase the concentration nitrite oxidizer bacteria. Before final optimization NOB in reactor #6 was 5.59 mgCOD/L, and after setting the DO to 1.5, the NOB can be increased to 11.79 mgCOD/L. More conversion to nitrate is needed since wastewater from reactor #6 will be returned to reactor #1 to be denitrified. By increasing the internal recycle ratio to 2, more flowrate is returned to reactor #1. Combined with higher available nitrate in reactor #6, it means that now there is more nitrate that can be denitrified.

The final effluent quality is summarized in table 8. Within 10 days of simulation, the daily average effluent quality has met the target effluent. For total-N, the daily maximum effluent quality met the target effluent at day 3, and ammonia at day 2.

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7 Table 8 Effluent Quality after Final Optimization

Parameter Average effluent quality (mg/l) Max effluent quality (mg/l)

COD 44.91 48.52

Total N - Day 1 - Day 2 - Day 3

8.99 8.53 8.49

10.69 10.04 9.98 Ammonia

- Day 1 - Day 2

0.75 0.68

1.02 0.94

Figure 4 Total N and ammonia effluent after final optimization

5.3 Assumptions and limitations

The main assumptions on this case is that the influent N to the treatment plant is constant at 45 mg/L. Therefore, this designated treatment plant can achieve the standard effluent when the nitrogen influent maximum concentration is at 45 mg/L.

6 Conclusion

Reject water contains high concentration of nutrients, especially nitrogen that will increase the organic loading in the main bioreactor. Side-stream treatment is beneficial to use in this situation to treat reject water before being recirculated into the main bioreactors. With side-stream removal, a higher treatment efficiency of total Nitrogen can be achieved. After final optimization, the effluent of the plant will have maximum concentration of COD, total N, and Ammonia of 48.52 mg/L, 10.69 mg/L, and 1.02 mg/L respectively. However, after 2 days of operation, the concentration of total N and ammonia will further decrease and always stay under the standard of 10 mg/L (total N) and 1 mg/L (Ammonia). By implementing side stream treatment, the power demand for aeration will decrease, and thus reducing the cost for power consumption.

7 References

van Hulle, S. W., Vandeweyer, H. J., Meesschaert, B. D., Vanrolleghem, P. A., Dejans, P., & Dumoulin, A. (2010). Engineering aspects and practical application of autotrophic nitrogen removal from nitrogen rich streams. Chemical Engineering Journal 162, 1-20.

BioWin Modeling and Simulation Report

CIE4703 Water Treatment