THE PROJECT - Cold Weather Nitrification
A City situated in the Northern Province of China has a population of 4.40.000 inhabitants. The city has severe winter weather during most of the time of the year. To reduce water pollution, the authorities have already planned the construction of a 10000 m3/day Sewage wastewater treatment plant for the city’s wastewater management. Originally the plant was designed based on the oxidation ditch pond variant of the Activated sludge process to meet only the BOD removal requirement for statutory compliance.
THE PROBLEM
However, due to changes in legislation, the plant is required to meet Chinese class I criteria of BOD, TSS, TN < 15 ppm, Ammonia < 5 ppm, and COD, 50 ppm for compliance. Due to extreme cold weather conditions, achieving COD < 50 ppm and Ammonia < 5 ppm poses a significant challenge.
As it is a well-known fact that every degree C of decrease in liquid temperature reduces the biological activity and thus removal rates of the activated sludge process. This reduction in the biological activity and removal kinetics of activated sludge required to double the volume of the existing aeration tank to compensate for the treatment requirement as per the new standards.
THE CHALLENGE: Cold Weather Nitrification of Municipal Wastewater
Due to inherent problems related to the cold weather conditions and space restrictions, the city was looking for a solution that could deliver the required results within the constraints with full utilization of the already under-construction plant.
Principle challenges associated with the project were:
- Very low liquid temperature (8-10 Degrees C) during the winter period resulting in lower biological activity of activated sludge
- Already plant was under construction and thus utilizing existing infrastructure and lack of further space availability were severe constraints
- Inlet load fluctuates a lot between spring and winter seasons due to specific water usage patterns which require to be treated to new stringent standards of BOD, TSS, TN < 15 ppm, Ammonia < 5 ppm, and COD < 50 ppm under all conditions.
THE SOLUTION: LEVAPOR-IFAS Upgradation of existing Activated Sludge Plant
Due to our experience with cold weather nitrification of municipal sewage wastewater, a hybrid activated sludge-based IFAS process using Levapor MBBR /IFAS media was suggested for the proposed modification of the plant.
After extensive trials at the site, conducted under different scenarios of inlet wastewater conditions of temperature, BOD, COD, TSS, and TKN, a design was implemented at the site considering the existing aeration tanks under construction.
During the trials, it was found that even at a shorter HRT of 3.5-3.8 hours, the required results can be achieved for the plant, and inlet wastewater treatment capacity can be also doubled to 20-22 000 m3/day if required. Already the client was planning to construct another 10,000 m3/day plant to meet the full treatment requirement, so it was decided to double the capacity also.
Based on the pilot trials and previous experience, the following design for the proposed project was suggested.
| Design Parameter | Value |
| Wastewater Flow | 20-22000 m3/day |
| Inlet BOD | 250-300 ppm |
| Inlet COD | 550-700 ppm |
| Inlet TKN | 25-65 ppm |
| Liquid Temperature | 8-10 degree C Min |
| Hybrid IFAS Reactor Volume | 3200 m3 |
| HRT | 3.5-3.8 hours |
| % Levapor Carriers filling | 12.5% |
| Levapor carrier total quantity | 400 m3 |
The Results
During October 2010, which is already winter in the city, the plant was commissioned after adding retention screens and 400 m3 Levapor MBBR-IFAS media to the existing Aeration Tanks.
Despite low temperatures, the plant achieved the COD reduction requirements within 2 weeks period, and after a 4-6 weeks period, stable nitrification was also established effectively achieving the required treatment goals as per Chinese Class I criteria.
Over the years of operation, the overall COD reduction at the plant has been stabilized to a much higher level than the conventional activated sludge process. Fig 4 and Fig 5 indicate the COD removal efficiencies and impact of temperature on the outlet COD values for the hybrid Levapor IFAS reactor. Despite fluctuations in the temperature as well as inlet COD, due to the superior performance of the Levapor IFAS process, the plant is consistently achieving outlet COD < 50 ppm throughout the year.
NITRIFICATION AND TKN REMOVAL:
Due to slow slow-growing nature of nitrifying bacteria and their sensitivity to pH, Salinity, Temperature, and certain pollutants, establishing the nitrification process in any activated sludge is a challenging task for the Biological Nutrient Removal (BNR) process.
However, due to its fine pore structure with the presence of activated carbon, Levapor MBBR-IFAS media provide very high surface area and high inner porosity which provides an ideal habitat for the nitrifying bacteria immobilization on the carrier surface in the biofilms.
Fig 5, represents the nitrogen removal trends at the sewage treatment plant after the addition of Levapor carriers in the reactor. Despite higher fluctuations in the inlet wastewater temperature and nitrogen load, the plant is achieving a remarkable level of nitrification delivering the required level of TN and Ammonia as per the Chinese class I criteria of Ammonia < 5 ppm and TN < 15 ppm.
BENEFITS
- Despite a lower retention time of 3.5 -3.8 hours, the plant is consistently achieving COD < 50 ppm and ammonia < 5 ppm as per the client’s requirements.
- Despite higher fluctuations in the inlet load and temperature, the plant is achieving the required standard under all conditions.
- Plant capacity has almost doubled and no further civil construction or space is required for further expansion and upgradation of the existing setup.
Amit Christian is a MSc graduate in Environment Science from Middlesex University, London, UK. He has been active in the field of water and wastewater treatment since 1998. He specializes in the design, engineering, and management of various biological wastewater treatments such as Activated Sludge Process (ASP), Sequencing Batch Reactor (SBR), Moving Bed Bio Reactor (MBBR), and Integrated Fixed Film Activated Sludge (IFAS). He has helped various Industrial and Municipal clients in troubleshooting, and optimizing their biological wastewater treatment processes to achieve the latest Stringent norms for Ammonia Removal.
