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Aerated Lagoons Primer Need to know more? Send us an email or feel free to search our online series with our give away search engine: |
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Very roughly, undesirable/excess wastewater constituents may be present in either or both of two forms, namely soluble and/or particulate, a cutting point being about 0.45 µm. To a certain extent, nonsoluble fractions can or ought to be removed by appealing to essentially physical pretreatment means, e.g. settling, flotation, screening. However, this first step leaves us with an at times sizable amount of material in soluble form that will still have to be dealt with. Because further appeals to physical processes will be of no avail (also law of diminishing returns), present environmental engineering practice brings in and banks on the phenomenal power of natural/biological processes. Wastewater treatment plants essentially replicate in somewhat controlled mode what has been going on in nature for ages: biological processes. In this way, "troublesome" soluble components are gladly gobbled up by living matter in specially conditioned "microorganism farms." Typical embodiments include attached-biomass type biotowers and suspended-growth aerated lagoons and activated sludge basins. Common to all approaches is growth and development of large microorganism inventories that will pick up soluble stream components, be it organic matter, nutrients like N and P, which in turn can and will be subsequently removed from said stream by ... physical means, e.g. settling, flotation, screening/membranes.
Broadly speaking, aeration systems employed in aerated lagoons are popularly classified as either surface aeration systems or submerged type aeration systems.
Typical examples of surface aeration systems include most frequently floating or pier-anchored mechanical type units, such as direct-drive, high speed units or gear driven, low speed units. Flow can be either upflow or downflow and either axial or radial/centrifugal.
A typical direct-drive, high speed unit consists of a motor, a fiberglass or stainless steel float and an intake/suction cone. The most common designs can be marine type impellers assisted with fixed/non-rotating diffusion heads or screw centrifugal, Archimedes type impellers. A good quality, high speed unit can and should deliver say about 2.4 lbs O2/hp/hr, +/-10%, in clean water.
A typical gear-driven, low speed unit consists of an electric motor, gearbox, relatively large diameter rotors (say up to 10' or 3.2m), spool and mounting plate for pier-mounted units Floating type low speed units include knocked-down, float platforms that can be easily assembled onsite. A good quality, low speed unit can and should deliver say about 3.5 lbs O2/hp/hr in clean water. .
Surface aerators are typically employed in the relatively shallower ponds, basins or tanks. Evaporative cooling does take place which may be undesirable or unacceptable in some contexts. Volatile organic compound stripping can be significant and/or again unacceptable
For best performance, surface aerator vendors frequently suggest recommended/minimum/maximum liquid depths for their standard units. If basin is too deep, the aerator may not be able to effectively pump up beyond a given depth thus resulting in idle pockets or even whole layers, at least as regards intended aerobic activity.
In order to be able to use them in [even] shallower basin anti-erosion assemblies must be incorporated.
As surface aerators pump up and splash out liquid, they induce very high velocity flows directly beneath them, and in some instances may cause damage to basin floor. In earthen or lined basins, aerator vendors usually recommend the use of bottom concrete pads directly below the units, although wastewater and concrete/other proposed material compatibility must be verified.
The most popular submerged type aeration systems include diffused aeration systems and submerged, turbine-type aeration and mixing aerator configurations.
Diffused aeration systems are frequently classified into two major categories according to the diffuser's pore/bubble size, i.e. fine-pore diffusers and medium/coarse diffusers.
Medium/coarse diffused aeration systems are used in foul-prone applications.
Both fine and coarse bubble diffusers can be used in retrievable racks/arrangements, diffuser banks or assemblies either sitting on basin bottoms or evenly suspended to overcome irregular, lagoon-type floors.
Submerged, turbine-type aeration systems include slow rotating bottom impellers coupled with grade level blowers. The submerged impeller draws liquid from the bottom for reactor mixing and effects oxygen transfer/bubble shearing Blower units provide air to the submerged turbine assemblies, (e.g. 35-40 SCFM per turbine share motor HP, ballpark 50/50 total HP split between blower and submerged turbine) via flexible hoses as needed to satisfy specific operating modes/targets, e.g. just mixing (off), anoxic stage, SBR phases, filamentous bacteria control.
Submerged jet aerators, i.e. basically a pump and submerged venturi-type diffuser, call for 8 to 10 m deep water levels. In this type of system, mixing and air supply can be operated independently of each other, i.e. pump only or pump and controlled introduction of pressurized air.
Aspiration type units provide good oxygen transfer but also cause a circular pattern of flow through the reactor. This circulation pattern is OK if the basin type requires circulation, such as oxidation ditches and facultative lagoons, but BNR reactors do not need this circulation. Aspiration type aeration devices also provide a high velocity jet that can cause erosion of the bottom or sides of the basin if the basin has a shallow depth or the unit is too close to the side of the berm.
Most types and brands are suitable for AS applications, but each has its own best applications. For example, brush aerators are best for oxidation ditches while fixed diffusers and surface aerators are best for conventional AS systems. The key is to size the unit properly for each application. Once OTR characteristics are established, the sizing is fairly straightforward. Other factors include alpha factor, impact of floc size and settleability, impact on effluent TSS, etc. The key phrase is "if properly sized/selected."
It is very frequent to employ treatment trains using lagoons in series. Another way to do this is by partitioning a large lagoon value into zones using floating baffles or curtain-like arrangements.
Microstrainers, ie microfilters, are not good for removing algae from lagoons without pretreatment. Pilot studies found very poor results with direct application to hollow-fiber microfilters.
Dissolved flotation units (DAFs) can be used for removing algae from lagoon effluents unless the application requires extremely low turbidity treated water. Even in the latter
case, DAFs would still be recommended, or ballasted flocculation, followed by microfilters. Placing high concentrations of algae directly on microfilters is possible, but produces a high solids load and requires frequent cleaning of the membranes and reduces the life of the membranes.
