Evaluation of Nitrification Process in Saline Wastewater Treatment by Hybrid Reactor and Its Comparison with Conventional Activated Sludge

Document Type : Research Paper

Authors

1 Lecturer at Shahid Beheshti University

2 Assistant professor of shahid beheshti university

Abstract

Introduction
Saline wastewaters contain mineral salts which are often produced by industries such as seafood processing, cannery, vegetables, and tanning. They are imported into the sewage collection network. In coastal cities, saline water consumption seems indifferent in household parts and highlevels ofground water, leaksaltwater, precipitation duringdifferentseasons,and sea level are the other reasons of the salinity of the wastewater.Saline wastewater biological treatment due to the harmful effects of salinity on microorganisms is difficult,because high salinity can causecell activity reduction. Regarding this matter, biological treatment is welcomed most in comparison with chemical or physical treatment because these systems are cost-efficient and compatible with environment. In addition,theyrequirelow-waste chemical materialsand canproduces less solid and require less energy. In the process of biological treatment, removing ammonia from wastewater is important because if the pollutants enter the waters of the acceptor, it would create problems such as toxic to aquatic organisms, depletion of dissolved oxygen water, creationof eutrophication, and increased use of chlorine. One of the important processes of ammonia removal is nitrification or the conversion of ammonia to nitrate. This processcannot be carried out effectively in wastewater treatment by conventional activated sludge methods. Because the salt concentration affects the osmotic and ionic composition and these two physical factors play an important role in the survival and reproduction of microorganisms.  On the other hand, the concentration of salt in the conventional processes would increase the buoyancy or float and removing sludge out of these systems is performed easily.Therefore, the age of proper sludge for nitrification cannot be supplied.To solve the mentioned problem, the hybrid growth method seems proper which causes the sludge to remain in the system longer, and it is expected that nitrification process works better in hybrid growth systems.Therefore, the present study is designed and performed by the aim todeterminethe efficiency of hybrid growth system in nitrification process in comparison with conventional activated sludge systems.
 
Material and methods
To perform this experimental study, a pilot in experimental scale was created and used. The features of the study are mentioned in the following.This pilot contains a raw sewage tank of 300 liters volume equipped with a stirrer and two biological unit reactors (conventional and hybrid activated sludge treatment systems), each with 34liters volume. It also has2 units of settling, each19 liters volume, and flow meters tomeasureairflow. It also contains2 dosing pumps for the injection of raw sewage and equipped with a compressed air compressor to supply the air required for reactors.
To accomplish this study, a real domestic sewage (wastewater treatment plant entrance martyr Beheshti town) was used. The variables involved include the concentration of ammonia, nitrate, dissolved oxygen, suspended solidin concentration of the liquid, hydraulic retention time and the rate of the incoming air. Titration method was used to measure the concentration of ammonia; Spectrophotometer was used to measure the nitrate, dissolved oxygen with help of electrode and to determineMLSS using the gravimetric method in accordance with standard procedures for testing of water and sanitation. In order to fix the rate of the flow of air in the reactor, two flowmeters were used.For fixingthe growth conditions of the hybrid, 45 percent of one of the reactors was covered with media polyethylene. The operation of the reactors was continuous. At the beginning of the pilot biomass of the reactors sludge return wastewater treatment plant by one-third the size of each reactor was catered and added to the reactor. Then, raw wastewater is inserted and when the pilot reached a stable condition, the involved variables were evaluated. In order to adjust the level of TDS in the amounts listed, pure sodium chloridewas used. Todeterminethe efficiency of the studied reactors in the nitrification process, three ammonia levels (17, 30 and 45 mg per liter) and three air intake rate (0.73,0.83 and 0.91) as well as three hydraulic retention time (5,4,3 hours) were considered. The reason forchoosing three mentioned air rates is to supply the dissolved oxygen at a concentration of 3 to 4 milligrams per liter as an optimal concentration for the hybrid growth. The SPSS software was usedto analyze data and Excel software was deployedtodrawgraphs.
 
Results and discussion
The results related to review of nitrification process for hybrid or conventional reactors show that in order to reach proper MLSSS concentration in reactors in conventional process of sludge return to 50 % of input rate, it was not necessary to return sludge inhybrid reactor and the proper concentration of MLSSS. Results of the present study indicate that when a hybrid system is deployed for saline wastewater treatment compared to conventional activate sludge systems, less air is required to maintain adequate dissolved oxygen concentration in the reactor. As it is shown in results (table 1),with fixed rate of input air to two reactors, dissolved oxygen concentration in hybrid reactors is higher than conventional reactors, in average 6.57.This issue in full-scale treatment plants can lead to a lot of savings in cost of air-supply, since a main part of the total cost of a municipal wastewater treatment is spent on air-supply of reactors.
 
Table1: The results of nitrification process in conventional and hybrid reactors





Influent


Reactor no.1:Conventional


Reactor no.2:Hybrid growth




NH4+
(mg/l)


Q air (l/min)


HRT
(h)


DO
(mg/l)


NH4+
(mg/l)


NO3-
(mg/l)


MLSS
(mg/l)


DO
(mg/l)


NH4+
(mg/l)


NO3-
(mg/l)


MLSS
(mg/l)






17


0.73


3


0.35


16.45


0.25


3975


2.25


0.87


31.45


3550




30


0.83


3


0.47


27.3


0.18


3120


3.12


0.93


57.64


3420




45


0.91


3


0.52


44.2


0.15


3350


4.45


1.51


84.12


3180




17


0.73


4


0.51


15.8


1.45


3150


2.96


0.22


33.77


4040




30


0.83


4


0.50


26


0.30


3550


3.26


1.4


61.41


4070




45


0.91


4


0.60


43


0.17


3000


4.81


1.25


92.5


4150




17


0.73


5


0.67


15


1.87


3450


3.13


0.75


36.2


4857




30


0.83


5


0.58


24.73


0.35


3084


3.52


0.15


64.15


4502




45


0.91


5


0.75


41.24


0.2


3818


4.97


0.95


95.74


4230





 
Another significant point in hybrid reactors is that although sludge does not reverse, MLSS concentration is maintained in an average of 4000 mg in per liter. While the MLSS amount in conventional reactor is supplied by the reverse of 50 percent of sludge. This issue is very significant economically because for reversing sludge to conventional treatment houses requiresa lot of money for pumping sludge and for its reverse which in hybrid systems can be saved. Moreover, in order to perform nitrification process completely in wastewater treatment systems, the proper age of sludge should be supplied which it was obtained in a system with hybrid growth. In a conventional system, due to high buoyancy and density in saline wastewater, adequate sedimentation of sludge does not occur. Therefore, in addition to the problems related to increased turbidity effluent output, adequate sludge supply is also a problem.. While in hybrid systems due to rare escape of suspended materials, supplying age of sludge is easily feasible, as thiswas proved in this study by the results. According to the results of the present study, nitrification process works better with hybrid system rather than conventional system. It is because of dissolved oxygen concentration and proper age of sludge. According to the results, DO concentration in hybrid systems is several times higher than conventional system. Therefore, ammonia removal and its transformation nitrate occurred successfully in hybrid systems. The reason is the prevention of present media in the system in contrast to oxygen exit out of reactor which leads to better mass transfer and maintains DO concentration in a higher level.
 
Conclusion
Results of the present study have indicated that nitrification process in hybrid growth system can perform better than conventional growth of activated sludge. Moreover, dissolved oxygen concentration remaining in hybrid reactor in a fixed rate of input air is higher than conventional growth.This matter can be very economical in wastewater treatment houses. In many southern cities, due to different reasons like saline water penetration into the network or saline water usage in residential units, sewage will have high dissolved solids, and nitrification process in these treatment houses faces many problems. Therefore, use of hybrid growth system is suggested inthe cities for better performances.

Keywords

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References

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Journal of Environmental Studies
Volume 42, Issue 1
June 2016
Pages 169-175

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