Case Study: Municipal plant
Client: Municipality of Marines-Valencia, Spain
Start-Up: May 2007
Capacity: 380 m3/d
In view of EU directives for effluent discharge, it was required to upgrade the local sewage plant in the town of Marines, Valencia (Spain). The technology chosen for the project was AGAR® (Attached Growth Airlift Rector) of Aqwise. La Entidad Pública de Saneamiento de Aguas Residuales (EPSAR) has promoted the implementation of the AGAR® system in the Valencian Region, whereas SAEM Enginyeria was chosen as a co-designer company for the system. For the installation of the process in the existing aeration tank, the plant had to be shut down for only five days. Effluent quality, as measured along two months following the upgrade project, complies with EU directives as planned.
Sewage from the small town Marines, in the Valencia region, Spain, is treated in a centralized wastewater treatment plant (WWTP). Secondary effluent from the plant is discharged into a dry gully near the plant. According to current EU directives regarding the collection and treatment of urban wastewater, in some cases WWTPs are required to remove nutrients (nitrogen and phosphorous) as part of their biological wastewater treatment. The Marines plant was only designed for removal of organic carbon, and was not equipped with the components necessary for nutrients removal. In addition, the plant occasionally receives instantaneous peaks of organic load from the local food industry.
Description of the original plant
The original plant included the following unit operations:
- Pre-treatment by mechanical screening with openings of 3 mm
- Biological treatment by extended aeration activated sludge process, in a single concrete tank. Air was supplied to the process through an array of fine bubble disc diffusers fully covering the tank bottom.
- Secondary settling in a rectangular settling tank, wherein, secondary sludge from the bottom of the clarifier was circulated back to the reactor, as part of the activated sludge process.
- Excess sludge from the clarifier was transferred to the thickener and belt filter.Shortcomings of the original design
Although the original plant was properly designed, it suffered from the following shortcomings:
- The process configuration was not suitable for nitrogen removal through nitrification / de-nitrification;
- In times of peak organic loads coming from a local canning factory, when sewage BOD5 levels would rise to more than 600 mg/l and COD to more than 1,000 mg/l, plant performance would deteriorate, resulting in poor effluent quality.
The above reasons indicated that a change in plant configuration was necessary in order to achieve the goal of biological nutrient removal while facing fluctuations in the strength of the incoming wastewater.
The selected solution
The method chosen for the upgrade was an AGAR® IFAS (Integrated Fixed-film Activated Sludge) system, based on the AGAR® (Attached Growth Airlift Reactor) technology of Aqwise. The AGAR® technology incorporates a bio-film process in a completely stirred tank reactor. The bio-film develops on plastic carriers which move freely in suspension in the reactor tank, using air supplied from diffusers for both mixing and oxygen supply.
In an IFAS system, a biofilm develops on plastic carriers, which are used in combination with conventional activated sludge. This combination takes advantage of the good settling properties of the activated sludge process with the increase in active biomass on the carriers. The result is sufficient nitrification and de-nitrification capacity without additional reactor volume, as well as short time for installation.
Design of the upgraded plant
The basin volume was divided into three consecutive stages using vertical partitions, which created a MLE process configuration: an anoxic pre-denitrification stage followed by one (or more) aerobic stages (Figure 1). The settled sludge from the clarifier and mixed liquor from the last aerobic stage were both recirculated to the anoxic stage.
Denitrification in the first anoxic stage utilizes the high BOD in the influent. Stage 2 of the reactor operates with activated sludge in order to achieve removal of any remaining BOD. Stage 3 of the reactor operates as IFAS, using both activated sludge and biomass carriers. While the activated sludge is aerated to maintain a low F/M ratio, nitrification is carried out on the biomass carriers. The aeration system was re-arranged by moving diffusers from the anoxic zone (which was aerobic in the original design) to the aerobic stages.
Effluent samples from the upgraded plant were analyzed regularly for two months after installation. The results show gradual and steady convergence towards stabilization of the BOD5, NH4+-Nand TSS values to below effluent quality requirements.
The municipal WWTP of Marines was successfully upgraded to comply with EU discharge regulations using an IFAS configuration of the AGAR® technology. The upgrade project did not require additional reactor volume, and was completed with only a very short plant shut down. Results following the upgrade show constant nitrogen removal and better BOD removal.
We would like to thank EGEVASA and Entidad Pública de Saneamiento de Aguas Residuales (EPSAR) for their support during the project.