Phosphorus

Phosphorus (P) is a key nutrient for the production of Biomass by photosynthesis. (Lawn fertilizer is mainly Phosphate and Nitrate.) So high levels of P are associated with excessive proliferation of aquatic weeds, algae, and cyanobacteria.

P accumulates from inflows that include runoff from agricultural land and discharges from wastewater treatment works. In addition, organic matter such as leaves fall into the lake and decompose in the sediments. Aquatic weeds, algae, and cyanobacteria tend to bloom in summer and then die off and sink to the bottom where they decompose and return P and other nutrients to the sedimentary stockpile.

Limnologists focus on managing the total “in-lake” phosphorus load, and this includes all P dissolved in the water as well as that stored in the sediments. So effective management of P loading must address both P in the water column and P in the sediment.

Popular strategies aimed at limiting further additional P inflows into eutrophic water bodies are therefore akin to closing door after the horse has bolted.

The graph shows the reduction of P levels in the water column at a large drinking water reservoir.

This lake is 500 acres with a maximum depth of 30 feet. The graph shows how the concentration of P in the sediment decreased every year after commissioning the solution.

Sophisticated bathymetric mapping allows the total volume of water in a water body to be calculated. The increase in water volume (after off-setting for changes in the water level) is due to the decrease in sediment and is, therefore, a measurement of sediment elimination by digestion of the organic fraction.

The graph shows surveys conducted in April 2018 before commissioning the solution and in October 2018 after the program had been running for four months.

The water volume in April was 893 322m³ (724.23 acre-feet) and in October 931 243 m³ (754.97 acre-feet). This equates to an increase in water volume and a reduction in sediment volume of 37 921 m³ (30.74 acre-feet). Had this volume of sediment been dredged out of the lake it would have required about 3 000 dump truck loads to haul it away.

Based on the sediment sampling and analysis, there was over 1,000 mg/kg of phosphorus in the sediment when at start-up. If we make a conservative assumption that the sediment weight before the system was started to be 1,000 kg/cubic meter, we end with the following equation:

37,917 m³ sediment reduced x 1,000 kg/m³ x 1,000 mg/kg = 37,917,000,000 mg P

37,917,000,000 mg P = 37.9 tons.

This would indicate that over 37.9 metric tons of phosphorus have been eliminated from the lake in four months.