Updated: July 2002Waste Water Nitrogen Removal System
1. System outline
Currently, removal of nitrogen is an important topic when planning a waste water treatment. There are various methods of removing nitrogen, each with advantages and disadvantages. However, the biological treatment method is used most commonly. With this method, organic nitrogen and ammonia nitrogen is converted into nitrous and nitrate nitrogen in an aerobic environment, and is dispersed into the atmosphere as anaerobic nitrogen gas. As there is no secondary pollution, this can be called an effective method. At sites subject to nitrogen regulations, BOD Policies are already in place, so basically, the problematic nitrogen is nitrous and nitrate nitrogen. Sinto has incorporated a special immobilizing carrier in our nitrogen removal system for nitrous and nitrate nitrogen. This device is introduced in this paper.
As previously explained, the nitrification process is carried out simultaneously with biological treatment of activated sludge processes, etc., to remove BOD. When BOD is removed but nitrification is not sufficiently carried out, measures can be taken to maintain the pH in the aeration tank and to increase the air flow rate.
The nitrified nitrogen is reduced and removed to nitrogen gas using denitrifying bacteria (expression iv), but if the denitrifying bacteria can be maintained at a high concentration, the volume load can be increased. The sponge-type cellulose carrier incorporated by Sinto has a very large surface area of 3000m2/m3, and the surface is coated with a positive charge, so it is easy to immobilize negatively charged denitrifying bacteria.
The immobilized carrier is filled in the reactor shown in Fig. 1, and stabilized denitrifying is continued with the four processes shown as 1) to 4) in the figure. 1) The carrier immobilized in the reactor is agitated to efficiently absorb the nitrified waste water. 2) Next, the denitrifying bacteria in the immobilized carrier decompose the nitrous and nitrate ion with the reaction given in expression iv. (Entry of nitrite ion is not a problem, and instead can simplify the reducing process.) 3) The immobilized carrier is sucked into the center of the stabilizer and is settled with a centrifugal process. 4) The settled immobilized carrier is discharged from the bottom of the stabilizer and is propelled against the wall with centrifugal force.
The nitrogen gas, which is a residue is compressed and pressed out from the carrier. Conventionally, this gas removal was insufficient, and obstructed the denitrifying reaction. However, this system features degassing by compressing the carrier with centrifugal force. Within the system, the denitrified processing water is reaerated (possible with approx. 30-minute retention time) to removed the excessively added methanol, and then the generated excessive sludge is settled to complete the process.
Fig. 1 Concept of Sinto denitrification reactor
2. Features and performance
As explained, the Sinto denitrification system has a major advantage in both the carriers and system. The system itself has the following features.
i) Space saving
Realizing a denitrification volume load of 4.5kg-m3N, the device space is approx. ½ to 1/10 of the conventional biological treatment method.
ii) Easy maintenance
As an immobilized carrier method is used, control of the sludge return is not required, and maintenance is extremely easy.
iii) Low running costs
The carrier consumption rate is small at approx. 10% of the filling rate even on an annual bases. Under normal usage states, the methanol, which is the hydrogen donor can be established with just slightly more additive than the logical value (expression iv), and little sludge is generated so the running costs can be suppressed.
3. Applicable fields and target Wastewater
Metal surface treatment waste water, salt bath waste water, etching waste water, semiconductor manufacture waste water, liquid crystal manufacture waste water, chemical manufacture waste water, foodstuff processing waste water, etc.