Hypolimnetic aeration

At the lower end of the plant, atmospheric air is introduced into the water using an ejector. This causes a combination of water and oxygen to be propelled upward in the upstream pipe. Upon reaching the end of the upstream pipe, the mixture enters the degassing chamber, where residual gases are separated from the oxygenated water. The gas is released into the atmosphere, while the oxygenated water continues its flow through the downstream pipe. The outlet is designed to provide a smooth, laminar flow and horizontal outflow into the hypolimnion.[2][3] Flow and mass transfer calculations conducted during the technical configuration can determine the optimal setup for a given system.

The system includes the following components:

• Floating tanks
• Upstream pipe (resembling a telescope)
• Degassing chamber
• Mixing device
• Suction fence
• Covering fence
• Downstream pipe
• Oxygen input
• Submersible pump with ejector
• Main ballast tanks

TIBEAN can be constructed using materials such as polyethylene, polypropylene, stainless steel, or an aluminum-manganese alloy.

Deep-water aeration offers substantial cost reduction and enables additional technical treatment of hypolimnetic water, particularly in the context of drinking water production.[1] As drinking water is typically sourced from beneath the thermocline in reservoirs, enhancing the quality of hypolimnetic water directly impacts the production of drinking water. Deep-water aeration can yield the following effects concerning compliance with drinking water regulations and the applicable limiting values:

The pH of drinking water has a threshold value of 6.5–9.5. pH values outside the neutral range (pH 6.5–7.5) are considered critical as they indicate the corrosion behavior of water. Slightly acidic water (pH 4–6.5) tends to corrode galvanized iron pipes, as well as copper and asbestos cement pipes,[6] in a process known as acid corrosion. Unprotected steel pipes are generally not suitable for use at lower pH values as they promote the removal of the zinc layer.[7] Natural cold waters typically exhibit a slightly alkaline reaction due to the equilibrium concentrations of dissolved carbon dioxide in the form of bicarbonate ions and carbonate ions, along with dissolved salts and gases. Higher alkaline pH values (pH 9–14) in the presence of oxygen can lead to oxygen corrosion. Buffer solutions are added to the raw water in drinking water production to prevent acid or oxygen corrosion. The pH-stabilizing effect of hypolimnetic aeration can reduce the need for these buffer solutions, thereby lowering operating costs.[1]

The threshold values for iron and manganese concentrations in drinking water are 200 µg/L and 50 µg/L, respectively. While iron and manganese are essential trace elements in drinking water, slightly elevated concentrations of these elements are undesirable from a technical and hygienic standpoint.[6] Under low oxygen conditions, iron and manganese dissolve as ions, with the majority existing as soluble ferrous or manganese compounds. At very high concentrations, the water may exhibit a yellow color. When aerated, the oxidation process results in the formation of ferric iron and manganese, causing red-brown and black precipitates, respectively. These precipitates can lead to water stains, turbidity, and laundry stains. They can also accumulate in pipes, leading to narrowing and deposition on fixtures. Iron levels above 0.3 mg/L and manganese levels above 0.5 mg/L can impart an unpleasant metallic taste.[6] By creating an aerobic environment in the hypolimnion, deep-water aeration facilitates the oxidation and precipitation of dissolved iron and manganese compounds before the water undergoes further treatment in a suitable facility for drinking water production. This approach helps reduce operating costs associated with the removal of dissolved iron and manganese compounds.

The quantity and mobility of iron species also affect the redox-controlled phosphorus cycle.[8] Divalent iron compounds originating from anaerobic sediment layers gradually diffuse and undergo oxidation at the boundary zone between aerobic water and anaerobic sediment. These compounds accumulate in the top sediment layer. The extent of this accumulation influences the effectiveness of the aerobic sediment-water boundary as a diffusion barrier for phosphate.[1]

As previously mentioned, deep-water aeration has the capability to significantly reduce nutrient concentrations. The presence of aerobic conditions promotes the processes of nitrification and subsequent denitrification, contributing to the removal of nitrogen from a system.[1] The oxidation of reduced substances like hydrogen sulfide and methane, both chemically and microbially, as well as the enhanced degradation of organic matter, can help mitigate sludge formation. Maintaining aerobic conditions in the deep water also plays a crucial role in reducing the redox-controlled redissolution of phosphorus from sediment and facilitating the reprecipitation of released phosphorus. Consequently, deep-water aeration offers the additional advantage of reducing costs associated with drinking water production by eliminating the need for denitrification stages or reducing the reliance on expensive flocculants.[1]