In this article we will discuss about:- 1. Types of Aerated Lagoons 2. Design of the Aerated Lagoons 3. Advantages.
Types of Aerated Lagoons:
Aerated lagoons are deep waste stabilization ponds in which sewage is aerated by mechanical aerators to stabilize the organic matter present in the sewage, rather than relying only on photosynthetic oxygen produced by algae. Thus aerated lagoons represent a system of sewage treatment that is intermediate between oxidation ponds and activated sludge systems.
Depending on how the microbial mass of solids is handled in the aerated lagoons the same are classified as:
(i) Facultative aerated lagoons and
(ii) Aerobic aerated lagoons.
(i) Facultative Aerated Lagoons:
Facultative aerated lagoons are those in which some solids may leave with the effluent stream and some settle down in the lagoon since aeration power input is just enough for oxygenation and not for keeping all solids in suspension. As the lower part of such lagoons may be anoxic or anaerobic while the upper layers are aerobic, these are termed as facultative aerated lagoons.
Further the facultative aerated lagoons are also known as partially mixed type aerated lagoons because these are operated at a low rate of aeration which is not adequate to keep all the solids in suspension.
(ii) Aerobic Aerated Lagoons:
Aerobic aerated lagoons are those which are fully aerobic from top to bottom as the aeration power input is sufficiently high to keep all the solids in suspension besides meeting the oxygenation needs of the system. No settlement of solids occurs in these lagoons and under equilibrium conditions the new (microbial) solids produced in the system equal the solids leaving the system.
Thus in this case the solids concentration in the effluent is relatively high and some further treatment is generally provided after such lagoons. If the effluent is settled and the sludge recycled, the aerobic aerated lagoon, in fact, becomes an activated sludge or extended aeration type lagoon.
A few typical characteristics of the above types of aerated lagoons are given in Table 15.2.
Facultative type aerated lagoons have been more commonly used the world over because of their simplicity in operation and minimum need of machinery. They are often referred to simply as ‘aerated lagoons’.
Their original use came as a means of upgrading oxidation ponds overloaded due to industrial wastes without adding to the land requirement. Further as the aerated lagoons are deeper than the oxidation ponds, and as they are artificially aerated, less land and less detention period are required for aerated lagoons as compared to oxidation ponds.
Flow conditions in aerated lagoons are neither ideal complete-mixing nor ideal plug-flow in nature. They are dependent on lagoon geometry and are better described by dispersed flow models of the type given by Wehner and Wilhem for first order kinetics and hence the design procedure given below is based on this dispersed flow model which takes treatability of the waste, temperature and mixing conditions into account.
The aerobic aerated lagoons have a complete-mixing regime and a slightly different mode of design is followed. However, as aerobic aerated lagoons have not yet been built in India (except one case) further discussion is limited to facultative aerated lagoons only.
Design of the Aerated Lagoons:
For facultative aerated lagoons, the dispersed flow model gives the relation between influent and effluent substrate concentrations, S0 and S, respectively and other variables such as the nature of the waste, the detention period and the mixing conditions, as shown in the Wehner-Wilhem equation given below-
A graphical solution of equation 15.14 is shown in Fig. 15.14 from which it is seen that prior knowledge of the substrate removal rate K as well as of the mixing condition likely to prevail in a lagoon is necessary to determine the efficiency of BOD removal at selected detention period. This is discussed below.
The mixing conditions in a lagoon are reflected by the term d which is known as the dispersion number and equals (D/UL) or (Dt/L2). It is affected by various factors. Observed results have shown the (D/UL) values to be in the approximate range given in Table 15.3 for different length-width ratios of lagoons.
By suitable choice of a lagoon’s geometry one can promote either more plug flow or more complete mixing type of conditions. Fig. 15.15 shows some different types of arrangements using baffles or units in series. In case of units in series, each unit may be well mixed with value of D/UL approaching 3.0 or 4.0 but overall the arrangements would give a relatively plug flow type arrangement.
Values of D/UL can be determined by conducting dye (tracer) tests on existing units using well known methods, but where D/UL values are required for design purposes prior to construction; they can be estimated either from lab-scale models or by using empirical equations available.
Low values of D/UL signify plug flow conditions and generally give higher efficiencies of substrate removal whereas the converse is the case with higher values of D/UL. However, process efficiency is not the only consideration; process stability under fluctuating inflow quality and quantity conditions, has also to be kept in view.
For municipal or domestic sewage, relatively plug flow type conditions (i.e., low values of DU/L) are preferred. In case of industrial wastes, relatively well mixed conditions (i.e., higher values of D/UL) may be preferred depending upon the nature of industrial waste; the greater the fluctuations in quality and quantity of industrial wastes, the greater the advantage in adopting well mixed conditions.
Lagoons are generally rectangular in shape though it is not absolutely essential. Natural land contours may be followed to the extent possible to save on earthwork. Lagoon units may be built with different length-width ratios and arrangement of internal baffles to promote desired mixing conditions. Lagoons may also be provided as two or three stage systems with the subsequent units placed at a lower level than the first if desired.
Construction techniques for aerated lagoons are similar to those used in case of oxidation ponds with earthen embankments. Pitching of the embankment is desirable to protect it against erosion. In cases where soil percolation is expected, suitable lining may have to be provided to maintain the design level in the lagoon and avoid pollution of groundwater.
Substrate Removal Rates:
As shown in Table 15.2 for facultative aerated lagoons the overall substrate removal rate constant K for sewage at 20°C, i.e., K20 varies from 0.6 to 0.8 per day (soluble BOD basis). At any other temperature T°C in lagoon the value of K, i.e., KT may be obtained from the following formula-
The average winter month temperature is critical for determining the detention time required. As stated earlier, the detention time to be provided in a lagoon can be determined from equation 15.14 or Fig. 15.14 for any desired efficiency for the computed temperature and mixing conditions in the lagoon.
The power input in facultative aerated lagoons has to be adequate only to diffuse dissolved oxygen uniformly in the system; no effort is made to keep the solids in suspension. Hence a minimum power level of 0.75 watts per m3 lagoon volume is adequate, but this should be checked with the aeration equipment supplier for its oxygenation characteristics and compatibility with proposed depth and shape of lagoon.
For treating domestic sewage the power requirement varies from 12 to 15 kWh per person per year or 2 to 2.5 hp per 1000 population equivalent. The oxygenation capacity of aerators is reported to range from 1.87 to 2.0 kg of oxygen per kWh at standard conditions for power delivered at shaft. Spacing of aerators should be adequate for uniform aeration all over the lagoon area without much overlap of the circle of influence of adjoining aerators as specified by the manufacturers. A minimum of two aerators would be desirable to provide to make up the total power requirement.
Aerators ranging from 3 to 75 hp are now readily available in our country. They can be either floating or fixed type. Floating aerators are mounted on pontoons (which should be corrosion-free). They have the advantage of being able to adjust themselves to actual levels obtained in the lagoons due to seepage and/or fluctuating inflows. Fixed aerators are mounted on structural columns and carefully levelled with regard to the outlet weir level to ensure required submergence of aerator blades to give the design oxygenation capacity.
The effluent is generally made to flow over an outlet weir. As the concentration of solids passing out in the effluent may be nearly the same as that in the lagoon the BOD corresponding to the volatile fraction of these solids (assumed as 0.77 mg per mg VSS in effluent) should be added to the value of the soluble BOD S obtained by use of equation 15.14 or Fig. 15.14.
Thus the final effluent BOD is given by-
Final BOD mg/l = S mg/l + (0.77) (VSS in effluent) mg/l
It is because of the suspended solids (expected to range from 40 to 60 mg/l in case of domestic sewage) in the final effluent that the total effluent BOD is difficult to reduce below 30 to 40 mg/1 in winter. At other times of the year BOD less than 30 mg/l may be possible.
This range of BOD is more than adequate for irrigation purposes, but for river disposal the applicable standards should be ascertained and design made accordingly. Where necessary, further reduction of BOD can be achieved either by a small increase in detention time or by more efficient interception of solids flowing out (e.g., providing deeper baffle plate ahead of outlet weir) or by provision of an additional treatment unit.
Nitrification is not likely to occur in aerated lagoons. Coliform removal ranges from 60 to 90% and it shows considerable seasonal variation.
Sludge accumulation occurs at the rate of 0.03 to 0.05 m3 per person per year as in the case of oxidation ponds and is manually removed once in 5 to 10 years and used as good agricultural soil. The depth of the lagoon may be increased a little to allow for sludge accumulation, if desired.
Advantages of Aerated Lagoons:
The various advantages of aerated lagoons are as indicated below:
(i) The aerated lagoons are simple and rugged in operation, the only moving piece of equipment being the aerator.
(ii) The removal efficiencies in terms of power input are comparable to some of the other aerobic treatment methods.
(iii) Civil construction mainly entails earthwork, and land requirement is not excessive. Aerated lagoons require only 5 to 10 percent as much land as stabilization ponds.
(iv) The aerated lagoons are used frequently for the treatment of industrial wastes.