Renovation and reconstruction of the Leitgiriai WWTP
Solution to which problem
Both the existing pre-treatment step and biological treatment step were technologically outdated and physically damaged from the long years of use.
Leitgiriai WWTP was built in 1991. It consisted of a grit chamber, a periodic operation ditch and pond – both with mechanical aeration, and a settling pond. Leitgiriai’s wastewater treatment plant (WWTP) was selected because it uses a typical biological treatment technology in the region, similar technological facilities are used in another four villages located in the territory of the coastal area: Girkaliai and Lapiai (Klaipeda district), Kaltinenai (Silale district), Pilsudai (Taurage district). In Lithuania, these facilities number about 20.
The new installation was designed for 163 PE (about 40 housholds), with an average daily inflow of 26 m3. The average influent concentrations to the system were 437.5 mg/L BOD7, 76.7 mg/L TN and 16.9 mg/L Ptot.
In the pre-treatment step old installations were dismantled and new hand skimmed screen for screenings and a sand/grit separator were installed. Also, a pumping station was designed for distributing wastewater flow during the wet periods when the inflow increases drastically. In these specific circumstances part of the wastewater is directed into the old reactor which is reconstructed into an equalization tank. After the excessive inflow is over the accumulated wastewater is returned to the treatment process.
In place of the old biological step (reservoir) a completely new bioreactor was installed with air blowers and an excess sludge tank. The biological process includes anaerobic, anoxic, aeration chambers and final clarification chambers. The whole system is designed to fit in one cylinder shaped compact tank, which allows minimizing the construction costs. Although the process is odourless, the reservoir is equipped with a cover to meet possible future greenhouse gas emission limits.
The anaerobic and anoxic chambers are divided into compartments, with different flow patterns to induce mixing (patented Vertical Flow Labyrinth system). In these chambers nitrogen and phosphorus removal takes place. The activated sludge-wastewater mixture from the anoxic chamber flows to the aeration chamber. At the bottom of the aeration chamber air diffusers are installed. The diffusers provide fine-bubble aeration ensuring the content is in suspension and the amount of oxygen necessary for biological processes is dissolved, perforated elastic membrane is used in the air diffusers, which allow a long-term operation. After the aeration chamber, the sludge-wastewater mixture flows to the final clarification chamber, which operates based on vertical settling process. The treated water flows through the collection system to the outlet.
The tanks of the biological reactors are designed to be installed partly below ground. The tanks of the WWTP are placed on a compacted gravel filling, on which is placed a reinforced concrete basement.
The excess sludge from the biological system is pumped out to the aerobic sludge thickening tank, from where it is periodically pumped out for transportation to further processing. Around 21 m3 of 2% TS sludge is generated by the system annually.
The installation runs fully automatically with a minimal demand for personnel intervention (besides periodical inspection of the system). The automation system enables constant surveillance and real-time adjustment of different technological processes.
The new installation removed the smell bothering the local residents, while also bettering the effluent quality discharged into the Leite river. Analysis showed the treatment of total nitrogen changed the most with the new installation, while removal efficiency for all three major indicators increased.
As the local residents were participating in the project, it also brought an increased sense of communication and community. The inhabitants received information about the conditions of the environment and water issues, which they consider important.