Aerated Pond

Increased mixing and aeration from the mechanical units means that the ponds can be deeper and tolerate much higher organic loads than a maturation or a facultative pond (see waste stabilisation ponds). The increased aeration allows for increased degradation and increased pathogen removal. As well, because oxygen is introduced by the mechanical units and not by light-driven photosynthesis, the ponds can function in more northern climates. Mechanical aeration enhances the treatment efficiency and reduces the required hydraulic retention time (HRT) for aerobic degradation of organics (ROSE 1999). It also increases pathogen removal because of the favourable effect of oxygen on solar water disinfection (CURTIS et al. 1992). The smaller area requirement means that it is appropriate for both rural, and peri-urban environments (TILLEY et al. 2008). However, the use of aerators also increases the complexity of the systems and technical material and energy is needed (ARTHUR 1983).

There are two types of aerated ponds:

• Aerated facultative ponds or lagoons (see also waste stabilisation pond systems), when it is desired to have a more aerobic system, compacter than normal facultative ponds, or when loads for conventional facultative ponds are too high.
• Completely mixed aerated ponds or lagoons.

Aerators used in partially mixed lagoons are generally placed on the lagoon surface and provide enough turbulence to satisfy the oxygen demand for aerobic oxidation, but they allow a sludge layer to form at the bottom of the pond. When energy is readily available, the water may be pumped through flow form cascades before entering the ponds for aeration. Aerators used in a completely mixed lagoon can be surface mechanical mixers or subsurface diffusers. They should provide enough energy to maintain the solids in suspension.

Completely mixed aerated lagoons are in essence activated sludge units without sludge return (ARTHUR 1983). As the sludge remains in suspension, effluents of completely mixed ponds require a post-treatment in a sedimentation pond. Just like for WSPs, the effluent of ponds can be reused in agriculture (e.g. irrigation)  or aquaculture (e.g. macrophyte or fish ponds) even though it has generally lower nutrient loads. However, the sludge which forms at the bottom of the aerated facultative lagoon or the sedimentation ponds (in the case of a completely mixed pond) needs to be dug out regularly. And before it can be reused in agriculture it requires a further treatment (e.g. through composting (compost chamber or large scale composting, anaerobic digestion or a constructed wetland).  

Influent should be screened and pre-treated to remove garbage and coarse particles that could interfere with the aerators. Because the aeration units mix the pond, a subsequent settling tank is required to separate the effluent from the solids.

The pond should be built to a depth of 2 to 5 m and should have a detention time of 3 to 20 days, depending on the treatment target.

To prevent leaching, the pond should have a liner. This can be made from clay, asphalt, compacted earth, or any other impervious material. A protective berm should be built around the pond, using the fill that is excavated, to protect it from runoff and erosion.

Aerated facultative pond/lagoon

The design of an aerated facultative pond is very similar to that of a facultative pond, with an aerobic zone close to the surface and a deeper, anaerobic zone. But there are no requirements in term of surface area as the process is independent of photosynthesis. The two main design criteria are HRT and depth. The HRT should be adopted in order to allow a satisfactory removal of BOD (biological oxygen demand) and is usually 4 to 10 days (VON SPRELING 2005) for organic loads of 20 to 30 g BOD/m3.day (SASSE & BORDA 1998). The depth of the pond should be planned keeping in mind the compatibility with the aeration system and the need of an aerobic layer of approximately 2 meters to oxidise the gases from the anaerobic decomposition of the bottom sludge. The amount of oxygen to be supplied by the aerators for the aerobic degradation/stabilisation of the organic matter should normally be equal to the total ultimate influent BOD (VON SPRELING 2005). However, such lagoons are generally designed using empirical methods: a HRT of 4 to 5 days results in 70 to 90% BOD5 removal in a partially mixed aerated lagoon by power requirements of 4 W/m3 (ARTHUR 1983).

Completely mixed aerated pond

Completely mixed aerated lagoons are essentially aerobic. The aerators serve not only to guarantee the oxygenation of the medium, but also to maintain the suspended solids (biomass) dispersed in the liquid medium. These systems are also called flow-through lagoons or CSTR (completely-stirred tank reactor) lagoons. Aerated ponds act similarly to aeration tanks in activated sludge processes. The main difference is that solids are not recirculated. Biomass and solids from the raw sewage are maintained together in suspension. This enhances the contact between bacteria contained in the biomass (responsible for the degradation) and the raw sludge to be degraded. Hence, the efficiency of completely aerobic ponds increases in comparison to partially mixed ponds and allows a reduction in volume.

Land requirements for this system are the smallest within ponds systems. The typical HRT of a completely mixed aerated lagoon is in the order of 2 to 4 days. This time is enough for an efficient removal of the suspended solids (VON SPRELING 2005). A HRT of 4 days, resulting in 70 to 90% BOD5 removal, requires about 20 W/m3 of energy (ARTHUR 1983). Aerated ponds have removal capabilities similar to facultative lagoons, except that nitrification of ammonia-nitrogen can be nearly completed in warm seasons, while cold weather will halt that process (U.S.EPA 2002). As the quality of effluent from these ponds is not satisfactory for direct discharge into the environment, completely mixed aerated lagoons should be followed by settling ponds. These may be either several short HRT ponds (i.e. 2 day), requiring frequent de-sludging (VON SPRELING 2005); or a single 10-day facultative pond with sufficient depth to allow long-term sludge storage. Aerators should be positioned carefully to avoid dead areas where solids are able to settle out already in the aerated pond. Small aerators rather than fewer large ones provide more evenly spread mixing, and rounded pond corners also help in avoiding dead areas (ARTHUR 1983).

The pond is a large expanse of pathogenic wastewater; health hazards can be caused by the aerosol effect releasing pathogens into the air (ARTHUR 1983); care must be taken to ensure that no one comes in contact with or goes into the water. The aeration units can be dangerous to humans and animals. Fences, signage, or other measures should be taken to prevent entry into the area.

A mechanically aerated pond can efficiently handle effluents with a  high concentration and can reduce pathogen levels significantly. But electricity service must be uninterrupted and replacement parts available to prevent extended downtimes that may cause the pond to turn anaerobic (TILLEY et al. 2008).

Investment costs are moderate to high, but expert design is required. Due to the large energy consumption for mixing and aeration, operation and maintenance is expensive. The aeration devices also increase the complexity of the unit and thus the vulnerability for technical failure (due to lack of replacement/spare parts or engineering skills). In consequence, the costs are generally higher than for WSP systems, depending on the local context and the availability of electricity. In some cases, solar-power driven aeration systems may be worth considering.

Permanent, skilled staff is required to maintain and repair aeration machinery and the pond must be desludged every 2 to 5 years. and the sludge needs to be either post-treated (e.g. [1182]-anaerobically digested], composted, [1868compos-incinerated]) or correctly disposed (TILLEY et al. 2008). The influents need also to be screened or settled as any large object could damage the aeration system. The pond itself must also be fenced off so no coarse objects can be thrown in.

Care should be taken to ensure that the pond is not used as a garbage dump, especially considering the damage that could result to the aeration equipment.