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Eutrophication, caused by excess nutrients like phosphorus and nitrogen, leads to harmful algal blooms and poor water quality in shallow freshwater systems. One method to combat this is adding iron to sediments to bind phosphorus and reduce its release into the overlying water. This thesis examines how effective and durable such iron treatments are, focusing on organic-rich sediments like peat soils. It addresses two main questions: how sediment properties and natural chemical processes influence the success of iron additions, and whether iron-rich water treatment residuals (Fe-WTR) from drinking water production can effectively reduce phosphorus release. 

Field studies were performed at three sites in the Netherlands: two peat polders and a shallow lake. In the peat polders, adding Fe-WTR proved effective by supplying large amounts of reactive iron that bind phosphorus, keeping the phosphorus trapped in the sediment even under low-oxygen conditions. At Lake Terra Nova, treatment with iron chloride (FeCl3) initially lowered surface water phosphorus concentrations, but levels later rose above pre-treatment values. This happened because organic matter binds iron in a way that allows phosphorus to be released quickly when oxygen drops during hot summers. 

The research deepened the understanding of the complex interactions between sediment and added iron and their effects on phosphorus dynamics. The type, crystallinity, and phosphorus content of added iron were shown to be crucial for treatment success, while the reliable dosage and long-term effectiveness remain difficult to predict. Fe-WTR shows promise but requires careful product selection and dosage, especially in organic-rich sediments.