Toa Vaca dam was built in 1972 to create a 247 acre raw drinking water reservoir in Puerto Rico. The maximum depth is about 174 feet. The intake tower for the water treatment plant has 6 intake gates, which are each about 30 feet apart. When built, it had a life expectancy of 300 years; however by 2009 it was highly eutrophic and fish kills were common. The plant was only able to use water from the top 2 gates without significantly increasing treatment costs. This created a significant problem during the dry season when reservoir levels typically drop below the first 2 gates.
In general terms, reservoirs have limited vertical mixing, and when combined with a high residence time and nutrients like nitrogen and phosphorous, both blue green algae and weeds find the perfect environment to dominate. Blue Green algae has no natural predators and can move up and down the water column, this together with their ability to produce energy through photosynthesis provides them with an important competitive advantage over other algae species.
The treatment of raw water coming from a eutrophic reservoir is a real challenge. Oxygen depleted zones at the bottom of the reservoir cause the migration of metals like manganese and iron and nuisance gases like hydrogen sulfide and ammonia to the water column. Once at the drinking water plant, this water requires an increase of chlorine and lime dosing. This leads to an increase in disinfection by products which are carcinogenic.
CLEAN-FLO designed a laminar flow oxygenation system for the reservoir and a treatment program for bioaugmentation. The system was installed in 2012. The bioaugmentation products were added approximately 4 months later. Our system consisted of 14 diffusers and 35,000 feet of airline.
A sampling program was established every two weeks in order to closely monitor the restoration process. Water quality parameters were analyzed at diffusers three (3), nine (9) and fourteen (14), at three different depths (surface, half depth and three foot from bottom) including: dissolved oxygen, oxidation reduction potential, hydrogen sulfide, nitrogen, phosphate, chlorophyll a, and manganese. During the startup of the diffusers a remote monitoring buoy was installed and operated in order to ensure that the emission of hydrogen sulfide gas was performed in a controlled way.
During the first year the water level dropped over 70 feet due to drought. The diffusers are more efficient with depth, so the reduction in depth significantly affected the rate of oxygenation of the entire water column. As the depth increased, the pace of restoration improved.
The benefits of restoring Toa Vaca reservoir are several and verifiable:
- Treatment plant cost reduction ( 25-35%)
- Reduction of sludge production (35-40%)
- Better taste and color of drinking water (no yellow water)
- Reduction of disinfection byproducts (TTHM)
- Reduced impact of water scarcity (rationing)
- Increased biodiversity
- No fish kills, increased fish catching
- Improved water infrastructure for a sustainable economic development
These 3 graphs show the reduction in total phosphorus, total nitrogen, and manganese at the bottom of the reservoir after the aeration system was started.