Stabilization lagoons represent one of the simplest possible options for wastewater treatment, with a level of engineering sophistication and cost below that of the preceding attached- and suspended-growth processes. As explained in an oft-quoted anonymous description—that "the design engineer drives a bulldozer and the rest is left to Mother Nature''—most lagoons are built as simple earthen basins, albeit with liners, and only limited operational oversight is provided. Given their simplicity, though, and ease of operation, lagoon systems are widely used in small rural communities and industries throughout the world.
There are three basic lagoon design options, including two that are solely aerobic or anaerobic, and a third, facultative version that has both overlying aerobic and underlying anaerobic layers. Whereas the aerobic and facultative lagoon options can be used for complete treatment of most municipal waste streams, anaerobic lagoon systems are more commonly used with high-strength industrial wastes (e.g., rendering plants, slaughterhouses, food processing residuals), and even then as a means of pretreatment prior to a following polishing step (e.g., followed by an aerobic lagoon, spray irrigation over land, or some other means of treatment).
All three of these basic lagoon design options have yet another subgroup set of design alternatives, depending on their projected organic loading rates and the means by which they will secure their necessary oxygenation and/or mixing.
Aerobic lagoons of the sort shown in Figure 16.33 rely on natural wind-induced mixing and/or surface aeration, plus algal photosynthesis, for oxygen resupply, and tend to be fairly shallow (with typical depths of about 2 m) and less heavily loaded. On the other hand, forced mixing and aeration using either mechanical agitation or subsurface compressed-air distribution (Figure 16.33) is also practiced with many aerobic lagoons, such that their design depths can be increased (to the range 3 to 4 m), along with receiving higher organic loadings. Anaerobic systems will be even deeper, often reaching depths of 4 to 5 m, to provide sufficient depths for the lower strata of the lagoon to fully reach a
point of oxygen depletion, and in many instances an overlying floating scum blanket will be allowed to collect on the lagoon surface to further constrain surface reaeration.
There is also a considerable range in the hydraulic detention times used with these various systems. Design HRTs below 1 week in length, at 3 to 6 days, may be used with highrate operations, or for anaerobic or facultative pretreatment units ahead of a second, downstream aerobic polishing lagoon. However, many stand-alone lagoons, particularly those not provided with supplemental aeration hardware, are sized to provide detention times in excess of a week and possibly extending to month-plus periods. Pretreatment focused largely on screening solids from the influent waste prior to entering the lagoon may also be provided, particularly in the case of high-strength industrial or commercial operations for which prescreening might effectively pull out a substantial level of incoming contaminant solids loading.
Example 16.3: Preliminary Stabilization Lagoon Design After completing the trickling filter and activated sludge preliminary designs, suppose that a third design overview is requested for the same Deer Creek, Illinois, community, with its 400 homes and 1600 residents. Given that their expected discharge point into a local creek has seasonal low-flow conditions that warrant high-level ammonia removal levels (i.e., to avoid summertime ammonia inhibition of sport fish in this Midwest city's regional area), it is assumed that a controlled-release, unmixed aerobic oxidation pond with a 6-month retention time will be required to handle the daily average wastewater flow of 605.7 m3/day and its commensurate total BOD loading of 121.2 kg BOD/day (Note: This design will be based on the community's full BOD load, since primary settling units are not typically included with these types of stabilization pond designs.)
Preliminary Design Details
Hydraulic loading, system sizing, and retention times
Design hydraulic retention time (HRT): 6 months Note: HRTs for these systems typically range from 1 to 6 months, depending on discharge restrictions into the receiving water body (e.g., a stream). Design reactor depth: 2 m
(Note: This depth is appropriate for unmixed lagoons; facultative and mechanically mixed aerobic lagoons have depths of 3 to 4 m, while anaerobic lagoons handling higher strength wastes are considerably deeper.) Projected reactor volume: (605.7 m3/day)(6 months) = 110,600 m3 Projected reactor surface area: (110,600 m3)/(1.5 m) = 55,300 m2 (Note: This area is equivalent to 13.7 acres, which would correspond to a per-area loading of approximately 116 people/acre.) Projected organic loading:
(121.2kg CBOD/day)/(110,600m3) = 0.0011 kg/m3 • day
1. This lagoon is rather large in surface area given its long HRT and shallow, unmixed depth; deeper, mixed ponds with lower HRTs often have per-capita loadings of 1 acre per 300 to 600 people.
2. As compared to either the activated sludge or trickling filter design options, this controlled (i.e., low-rate seasonal) discharge oxidation pond design would have a physical footprint roughly 1000 times larger.
3. Although it does require significantly more space, this type of facility should be less costly to build and apt to require less operational attention, although the stability of this system's effluent quality will probably be much lower.
To some extent, it is reasonable to classify these lagoons as quasi-suspended-growth systems given their content of suspended bacterial and algal biomass, although in this case the level of entrained biomass concentration tends to be several fold lower (i.e., generally measured in 100s of mg/L as opposed to 1000s). At the same time, most aerobic and facultative lagoon volumetric organic loading rates (ranging from 0.001 to 0.01 kg/ m3 • day) are also much lower than the more advanced attached- and suspended-growth processes, such that the biomass found in these lagoons tends to have a greater proportion of higher life-forms (e.g., scavenging protozoans, rotifers) and a lower net mass fraction of lower bacteria than what would be seen in a more highly loaded attached- or suspended-growth process.
Yet another unique aspect with open, aerobic lagoons (and to a lesser degree in mixed lagoons, due to the higher entrained solids levels and lower opacities) is that they have considerably more involvement with algal growth, due to the reduced density of their suspended solids and the fact that light can penetrate much deeper than is the case with most other waste treatment systems. With the incoming waste stream continuously introducing a fresh supply of readily available nutrients, algal and other small-scale plant growth can undergo large seasonal swings and will have a correspondingly significant impact on suspended solids levels in the effluent as well as the soluble oxygen and carbon dioxide levels.
The preferred circumstance for lagoons in terms of their algal composition it that of having suspended, submerged algal cells, where their metabolism often has a distinct, diurnal impact, resulting in sizable swings in dissolved oxygen and pH [i.e., higher DO and higher pH during the day, when algal photosynthesis and oxygen release offset bacterial respiration (at the same time, algae also removes CO2); lower DO and lower pH at night, while both bacteria and algae are respiring and jointly releasing CO2]. These algae are also effective scavengers of soluble nutrients, including both nitrogen and phosphorus, assimilating them routinely into solid-phase algal cell mass and then removing them once these cells die and settle onto the lagoon bottom.
However, there is also a smaller number of lagoons whose algal or plant content is established in a free-floating fashion, such as that of the classic duckweed plant form shown in Figure 16.34, where their presence tends to result in a more extensive mat stretched across the lagoon surface. These mats may appear quite green and healthy, but their activity and presence are somewhat contrary to conditions normally considered optimal for these lagoons. Free-hanging duckweed roots, called fronds, extend only a few centimeters in depth, and they contribute little, if any, dissolved oxygen via photosynthesis. Even worse, they physically block wind-induced surface reaeration, to a degree that their subsurface conditions are more likely anoxic to quasi-anaerobic. BOD and suspended solids removal can still be secured in these matted lagoons, albeit with
lower loading rates, and nutrient uptake will still occur, but effective long-term operation may require surface harvesting measures.
As for the issue of exactly what happens to the accumulation of dead algae, bacteria, and other waste solids once they settle and accumulate on the bottoms of these lagoons, this topic still represents a highly variable circumstance for most lagoons. Ideally, the input of biodegradable solids settling into a lagoon's bottom sludge blanket would then undergo decay at a rate not much different from that of their addition. If, indeed, this rate of accumulation were to be low, the sludge blanket depth would not increase, there would be no need to clean (i.e., remove sludge from) the lagoon periodically, and there would be no reduction over time in the lagoon's active (as opposed to settled sludge) volume. Unfortunately, the history with most lagoons is that they tend to accumulate settled solids at a rate that warrants their cleaning at multiyear to decade-long intervals, without which there would be a debilitating, progressive drop in effective lagoon volume.
Of course, there is also a negative side to large, exposed lagoon surfaces, particularly in regions faced with colder seasons, in that they will experience sizably higher levels of evaporative cooling that could lead to significant drops in temperatures during cold-weather periods. In turn, these reduced temperatures will then affect bacterial efficacy in the lagoon, particularly that of ammonia oxidation by highly sensitive nitrifying bacteria. From a regulatory perspective, this circumstance of decreased nitrification during cold-weather periods is most important for lagoons that discharge into creeks and streams with a high degree of variation in their seasonal flow, where low-level dilution of a highlevel effluent ammonia discharge might then cause downstream ammonia toxicity problems for fish. In fact, many sport fish, such as trout, which might be found in streams, rivers, and lakes downstream of these lagoons, are highly sensitive to quite low ammonia levels, to an extent where 0.1 mg NH3-N/L concentrations might incur serious, possibly fatal, stress. To resolve this problem, therefore, there is yet another lagoon option which incorporates a controlled-discharge release, with far longer HRTs extending up to, and even beyond, 6 months in time, to ensure that the effluent release can be delayed until the receiving water body provides an acceptable level of dilution that would negate this type of ammonia toxicity concern.
Lagoon effluent quality as a whole is not usually comparable to that of the more advanced waste treatment processes (e.g., activated sludge), with effluent BOD levels typically in the range 40 to 60 mg/L and even higher solids levels, typically 60+ mg/L. Algae represent a large fraction of these solids, however, and as a result, regulatory criteria levels with lagoon suspended solids concentrations are either set higher to accommodate this circumstance or filtration hardware (i.e., mechanical screens or sand, etc. filters) will need to be installed if deemed necessary to remove the latter solids prior to discharge. Therefore, disregarding discharge sites whose low-level dilution might incur nitrification concerns, lagoon effluents are commonly considered acceptable for general discharge.
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