Sequential Application of Different Types of Coagulants as an Innovative Method of Phosphorus Inactivation, on the Example of Lake Mielenko, Poland

The process of accelerated eutrophication forces the search for innovative, effective methods to restore the quality of surface waters. This study was conducted on shallow, urban Lake Mielenko (Maximum depth 1.9 m; Mean depth 1.3 m) in the context of implementing a new, sustainable method of lake restoration, i.e., phosphorus inactivation by sequential application of two types of coagulants. Approximately 9.9 tons of polyaluminium chloride (trade name PAX 18) were introduced into the profundal zone of Lake Mielenko, and 9.0 tons of iron chloride (trade name PIX 111) in the coastal area. The applications were divided into two spring and two autumn stages. Before restoration, the mean P min. concentration in Lake Mielenko water was 0.031 mg P/L, and TP was in the range of 0.091 to 0.346 mg P/L. After restoration, the average content of P min. was 0.007 mg P/L (a decrease of 80%), and the average value of TP was 0.096 mg P/L (a decrease of 72%). The obtained results indicate that phosphorus inactivation does not change nitrogen compounds’ content. However, due to the application of coagulants, P content decreased, i.e., the main factor limiting photosynthesis, which resulted in a significant decrease in primary production in Lake Mielenko. Before restoration, the average content of N-NH 4 was 0.100 mg N/L, N-NO 3 —0.145 mg N/L, N org. 1.70 mg N/L, and TN—1.86 mg N/L. After restoration, the average content of N-NH 4 was 0.096 mg N/L, N-NO 3 —0.123 mg N/L, N org. 1.28 mg N/L, and TN—1.50 mg N/L. This also resulted in a reduction in the N org. content, as well as a reduction in the amount of chlorophyll a (from 30.51 to 13.41 mg/m 3 ), organic compounds (BOD 5 from 8.9 to 4.6 mg O 2 /L), and an increase in water transparency to the bottom (up to 1.45 m on average). The results obtained in Lake Mielenko indicate that the innovative method of phosphorus inactivation, which involves the sequential application of two types of phosphorus-binding preparations, is an excellent solution that ensures higher ecological safety in the coastal areas of the reservoir and also allows for a significant reduction in restoration costs.