Deep water aeration with hypolimnion aerator LIMNO

Restoration of lakes using deep water aerator – rescue for over fertilized and polluted lakes

Our lakes and waterways suffer from pollution. Also Drinking water lakes and reservoirs can be affected by inflow of phosphorus and nitrogen. In addition, airborne eutrophication contributes to the pollution. All thusly influenced waterbodies need external help. Oftentimes, the problem manifests itself in oxygen depletion. If nothing is undertaken, conditions will continue to worsen, and no oxygen will be available in the sediment and above. Hence, all biological life is threatened. The hypolimnion aerator LIMNO offers an effective treatment for lakes that suffer from oxygen depletion.

Oxygen as characteristic factor

Oxygen as gas has only a limited solubility in water. Factors that influence this solubility are, amongst others, water temperature and atmospheric pressure. A healthy lake shows a dynamic equilibrium between oxygen feed through diffusion at the surface, photosynthesis and oxygen consumption through biological decomposition of organic material. In temperate climates, and especially in stratified lakes and reservoirs, the distribution of oxygen depends on the characteristics of the lake such as: ratio of volume of warmer surface water (epilimnion) to volume of deep water (hypolimnion). Observing the concentration of oxygen and its temporal and depth distribution provides a good picture of the overall health of a lake.

Diffuse nutrient sources, for example agriculture or influx of municipal and industrial wastewater, can provide a nutrient supply that exceeds the oxygen supply. Conditions turn critical when oxygen is depleted in the hypolimnion: Anaerobic, microbial processes (biochemical processes under exclusion of oxygen) are started. These reduction processes create methane and hydrogen sulfide: inorganic nutrients like phosphorus and nitrogen are solved in the water and later on, in the circulation periods, dispersed in the lake. The LIMNO aerator is designed to counteract this harmful mechanism. This deep water aerator feeds oxygen to the hypolimnion without destroying the thermal stratification of the lake. The oxygen level is kept at a healthy ratio throughout the entire stagnation period, which minimizes diffusion processes of nutrients from the sediment.

The LIMNO aeration system

The LINMO system has been developed for stratified lakes and reservoirs.

Functional principle of the aerator

The LIMNO aerator consist of two concentric hoses that are connected by radial walls and covered by a spherical hood.

The outer hose has several outlet nozzles at its lower end. A pipe for used air connects the top hood with the atmosphere. The unit is permanently fixed to the ground by concrete weights and nylon tethers that are attached at the outer hose and lower ring frame. The enclosed air cushion in the hood keeps the unit upright when in use as well as in standstill.

A compressor on shore provides the aerator with compressed air through a hose. A primary distributor, located under the suction cone, disperses the compressed air into finest bubbles. Those rise through the inner hose and create an upward flow (airlift pump principle).

Oxygen transport follows the creation of a high specific interface and turbulent flow between the air bubbles and the water.

The flow rate is reduced once the water-air mix crosses the upper rim of the inner hose. The excess air bubbles can escape and are led through the used air pipe.

Oxygen rich, bubble free water flows to the bottom of the outer hose and leaves it in form of a water jet to be dispersed in the hypolimnion.

Depending on the construction of the device, compressed air can also be led through a secondary distributor. It is a ring located in the bottom of the unit between the hoses. The bubbles from this distributor meet the downward stream and enrich it further. This air volume stream is optimized, so that the necessary ascend speed is not undershot.

The excess compressed air from the secondary distributor is also caught in the used air pipe and released into the atmosphere. The pipe is stretched by a buoy at the surface.

In conclusion, oxygen depleted or free hypolimnion water is sucked through the aerator, where it is enriched with oxygen and then dispersed horizontally into the deep water.

Mechanical properties

The LIMNO aerator is completely made of flexible materials. This simplifies transport and installation. All unit components consist of non-corroding materials, mainly plastics. The hoses, walls, hood, suction cone and outlet nozzles consist of special-polyester fabric. This material is in accordance with following standards: BS No. 3424, DIN 53352 and 53354, Fed-Std-5041 and 5100.

All seams are high frequency welded and enforced with additional strips. The ring frames (both upper and lower), used air pipe and distributors are made of polyethylene (PE). Depending on the diameter, the pressure hoses are equipped with a lead wire that is coiled around them (small diameters), or anchored to the ground using concrete weights and stainless steel bands.

LIMNO fabric data outer area inner hose and radial walls
Specific weight 1000 g/m2 670 g/m2
Tensile strength 4400 N/50 mm 3000 N/50 mm

Capacities and dimensions

All LIMNO units are constructed according to the customer´s specifications to fit the requirement of the specific project. Following table provides an overview of the capacities of the LIMNO aerators. Larger units can be planned and built if needed.

Range
Oxygen transfer rate [kg/d] 100 – 1600
Pressured air consumption [l/s]
(at 1013 mbar)
7 – 112
Diameter [m] 2 – 8,8
Weight [kg]
(w/o anchors)
250 – 1300
Weight of anchors [kg]
(under water)
350 – 4500

Installation

AGO Hydroair offers to lead the project and have the LIMNO units installed by its personnel. The equipment is delivered to shore. The LIMNO unit is folded and packed in a box. A forklift, or similar, is used to unload and lower the bucket shaped concrete weights into the water, where they float at a depth of one meter. The anchor weights are transported to their position by boat and then filled with water and lowered. The LIMNO unit is unfolded on shore and equipped with extras. Then it will be brought into the water, moved to its position, attached to the anchors and, with the help of a diver, pulled down and secured. The pressure hose is delivered rolled up in a coil. It is weighted down by either lead weights or concrete stones. The unit is ready for use after being connect to the compressor and a system check.

Operation

The LIMNO unit is used after the circulation period in spring, to feed the rising demand of oxygen in the hypolimnion.

The LIMNO aerators are usually used throughout the entire stagnation period in summer. The system is switched off with the beginning with the autumn circulation. Though, it can always be put to use again, should it be required in winter. The system´s construction enables the use in winter without obstruction of the ice layer. The enrichment capacity can be regulated according to the varying oxygen consumption. The desired oxygen concentration is reached by reducing the amount of pressured air, or by switching off single units of the system.

Pressured air supply

MCR-technology and compressors are delivered as compact units. They are installed in an accessible container near the shore. The container is designed and placed in accordance with the required guidelines. By painting and planting, the container can be neatly integrated into the landscape.

Pressured air is provided by a compressor. The required pressure equals the sum of hydrostatic pressure at the distributors and the drop in pressure in the supply hoses. The air must be oil free. Therefore, only oil-free compressors that need a minimum of maintenance are used. Since the hoses and supply pipes are made of PE, it might be necessary to install an aftercooler.

Surveillance

Pressured air flows from the compressor through a distribution pipe (supply pipe or hose) to each LIMNO unit that is connected to it by an individual hose. There are flow measuring devices, precision pressure gages and control valves for every single device, which allow for easy monitoring and operation. If desired, further control options like remote monitoring can be realized.

Tiefenwasserbelüftung Limno_kl_30
Belüftung des Tiefenwassers bei Erhalt einer intakten Schichtung

Change of quality of deep water through aeration with LIMNO

According to the characteristics of the ecological system, different lakes will show variant results after aeration of the hypolimnion. A eutrophic lake is different from a reservoir rich in iron and manganese. Nonetheless, the following graph shows universally valid and typical results.

Prinzipdiagramm zur charakteristischen Parameterentwicklung während der Belüftung
Prinzipdiagramm zur charakteristischen Parameterentwicklung während der Belüftung

The LIMNO aerator will be run at full capacity when used in a lake completely depleted of oxygen. A quick rise in oxygen concentration will be noted in the water. The system´s pressure capacity will be reduced once the needed concentration is reached in order to maintain it.

There is usually a correlation between the rise of oxygen level and an immediate drop of the iron content. This initial reduction is interpreted as a result of precipitation of ferric hydroxide with adsorbed phosphates. An additional, slower decrease of phosphate levels, following the initial quick reaction, can be explained with the oxidation of the sediment surface.

Deep water aeration triggers a biological reaction promoted by microorganisms turning ammonia to nitrate.

The reduction of iron in combination with phosphate has been mentioned above. The iron content is far lower in aerated lakes. Manganese has a higher solubility than iron, but it reacts in a similar fashion. Concentration of both metals is pushed to nearly zero in reservoirs where the LIMNO system is used. The result is better drinking water quality and lower treatment costs.

The normal variation of these parameters is large due to meteorological and hydrological factors as well as irregular inputs of nutrients. However, it was observed that through the use of the LIMNO system, chlorophyll levels dropped and a better water transparency was achieved.

List of references concerning LIMNO deep water aerator

Project client specification
Lake Tegel Senate for city development, environmental protection and technology Berlin 4500 kg oxygen per day
Flughafensee Senate for city development, environmental protection and technology Berlin 450 kg oxygen per day
Lake Gross-Glienicke Senate for city development, environmental protection and technology Berlin 1800 kg oxygen per day