With depleted underground water sources and lacking other surface water sources, Jiaxing City, China, needed to construct a new water treatment plant to purify polluted water from the local Yen Ping Tung River in order to meet a growing demand from local industry and an expanding population. Monitoring of river water quality revealed that it was moderately contaminated with ammonium, oxidized nitrogen, organics, iron and manganese. Categorized as Class 4/5 in the potable water standard, the river water was not ideal as a source for potable water. Conventional chemical and physical treatment methods were not cost effective in removing ammonium nitrogen and organics from the river water, so a biological treatment step was considered for pre-treatment.
A pilot study was arranged to verify the suitability of TETRA® SAF™ technology for this application, and a 60-day trial was undertaken in the early spring of 2004 to determine the performance of the SAF process.
The results from the pilot study are summarized below:
Water temperature |
11 - 18 degrees C |
NSAF Influent NH4-N |
10.1 mg/l (average) {range: 7 - 14 mg/l} |
NSAF Effluent NH4-N |
0.19 mg/l (average) |
Required effluent NH4-N |
< 0.5 mg/l |
NSAF Influent BOD |
12.26 mg/l (average) |
NSAF Effluent BOD |
2.15 mg/l (average) |
Required effluent BOD |
< 3 mg/l |
These results demonstrate that TETRA SAF is a suitable technology to pre-treat moderately contaminated river water (Class 4/5) to produce a good quality source (Class 1/2) for the potable water treatment works. As a result of the pilot trial, a full scale TETRA SAF plant will be completed in 2007 to process up to 25,000 m3/day of river water.
SAF Process Details
The TETRA SAF is an up-flow, fixed-film biological reactor. Biochemical oxygen demand (BOD) and ammonium-nitrogen (NH4 - N) are removed from river water by the process of biological oxidation. For water to be recycled for potable use, any ammonium-nitrogen present will increase the chlorine requirement for disinfection.
Fine-screened (6mm) river water and process air are fed into the bottom of the reactor, which has been designed for even distribution of air and feed as it flows upward through the media. The process air provides a suitable aerobic environment for the biological reactions to occur and is typically provided by duty/standby fixed speed blowers. The only control requirement is for auto-rotation of duty blowers and switch-over on detection of blower failure.
As the river water passes over the gravel media, ammonium-nitrogen and BOD are converted into biological solids by the action of the bacteria (oxidation). These solids are not captured and are continuously released from the media bed; therefore, no backwashing is required.
The only maintenance requirement is a periodic flush using SAF influent flow to remove any settled solids or debris from the influent pipe-work. This is typically carried out for about 3 minutes every other day.
NH4 - N Removal
Nitrification or "ammonium oxidation" is a two-step respiratory process in which bacteria oxidize ammonium (NH4) to nitrite (NO2) and nitrate (NO3). Simplified equations for these processes are as follows:
2NH4+ + 3O2 = 2NO2- + 2H2O + 4H+ (Eq. 1)
2NO2- + O2 = 2NO3- (Eq. 2)
Nitrosomonas (Eq. 1) and nitrobacter (Eq. 2) are the main organisms responsible for these steps. Overall these bacteria, present in the tertiary plant, convert the ammonium into nitrate.
Concentrations of nitrate are also normally expressed as mg/l of nitrogen. This means that when the bacteria remove 1 mg/l of ammonium nitrogen they produce 1 mg/l of nitrate nitrogen. As a by-product of ammonium oxidation, the bacteria also produce a small amount of acid which is normally neutralized by natural alkalinity present in the sewage. The bacteriological rate of conversion is much slower in cold conditions; therefore, effluent ammonium-nitrogen concentrations may be higher in cold weather.
Conclusion
The advantages of biological treatment offered by submerged aerated filters have become widely accepted and, as a result, the technology has become established for wastewater treatment across the world. The pilot study in Jiaxing City, China also demonstrated the TETRA® SAF™ bioreactor was very effective in removing NH4-N and organics from the polluted river water and transforming it to meet the required potable water standard. The SAF bioreactor is proven to be a very robust solution for pre-treatment of river water, facilitating water re-use where water shortages are a perilously common problem.
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