In the Midwest , oxidation ponds are used extensively for the treatment of domestic sewage from suburban areas .
The high cost of land and a few operational problems resulting from excessive loadings have created the need for a wastewater treatment system with the operational characteristics of the oxidation pond but with the ability to treat more organic matter per unit volume .
Research at Fayette , Missouri on oxidation ponds has shown that the BOD in the treated effluent varied from 30 to 53 mg/l with loadings from 8 to 120 lb .
Since experience indicates that effluents from oxidation ponds do not create major problems at these BOD concentrations , the goal for the effluent quality of the accelerated treatment system was the same as from conventional oxidation ponds .
Recent studies by Weston and Stack had indicated that a turbine aerator could be added to an oxidation pond to increase the rate of oxygen transfer .
Their study showed that it was possible to transfer 3 to 4 lb of oxygen / hr. / hp.
O'Connor and Eckenfelder discussed the use of aerated lagoons for treating organic wastes .
They indicated that a 4-day retention , aerated lagoon would give 60 to 76 per cent BOD reduction .
Later , Eckenfelder increased the efficiency of treatment to between 75 and 85 per cent in the summer months .
It appeared from the limited information available that the aerated lagoon might offer a satisfactory means of increasing the capacity of existing oxidation ponds as well as providing the same degree of treatment in a smaller volume .
Red Bridge Subdivision
With the development of the Red Bridge Subdivision south of Kansas City , Missouri , the developer was faced with the problem of providing adequate sewage disposal .
The sewage system from Kansas City was not expected to serve the Red Bridge area for several years .
This necessitated the construction of temporary sewage treatment facilities with an expected life from 5 to 15 Aj .
For the initial development an oxidation pond was constructed as shown in Figure 1 .
The oxidation pond has a surface area of 4.77 acres and a depth of 4 Aj .
The pond is currently serving 1,230 persons or 260 persons per acre .
In the summer of 1960 the oxidation pond became completely septic and emitted obnoxious odors .
It was possible to maintain aerobic conditions in the pond by regular additions of sodium nitrate until the temperature decreased and the algae population changed from blue-green to green algae .
The anaerobic conditions in the existing oxidation pond necessitated examination of other methods for supplying additional oxygen than by sodium nitrate .
At the same time further expansion in the Red Bridge Subdivision required the construction of additional sewage treatment facilities .
The large land areas required for oxidation ponds made this type of treatment financially unattractive to the developer .
It was proposed that aerated lagoons be used to eliminate the problem at the existing oxidation ponds and to provide the necessary treatment for the additional development .
The lack of adequate data on the aerated lagoon system prompted the developer to construct an aerated lagoon pilot plant to determine its feasibility for treating domestic sewage .
The pilot plant was a circular lagoon 81 ft in diam at the surface and 65 ft in diam at the bottom , 4 ft below the surface , with a volume of 121,000 Aj .
The side slopes were coated with fiberglas matting coated with asphalt to prevent erosion .
The pilot lagoon was located as shown in Figure 1 to serve the area just south of the existing housing area .
The major contributor was a shopping center with houses being added to the system as the subdivision developed .
The pilot lagoon was designed to handle the wastes from 314 persons with a 4-day aeration period .
Initially , the wastewater would be entirely from the shopping center with the domestic sewage from the houses increasing over an 18-month period .
This operation would permit evaluation of the pilot plant , with a slowly increasing load , over a reasonable period of time .
The pilot plant was equipped with a 3-hp. turbine aerator ( Figure 2 ) .
The aerator had a variable-speed drive to permit operation through a range of speeds .
The sewage flow into the treatment plant was metered and continuously recorded on 24-hr. charts .
The raw sewage was introduced directly under the turbine aerator to insure maximum mixing of the raw sewage with the aeration tank contents .
The effluent was collected through two pipes and discharged to the Blue River through a surface drainage ditch .
Composite samples were collected at weekly intervals .
The long retention period and the complete mixing concept prevented rapid changes in either the mixed liquor or in the effluent .
Weekly samples would make any changes more readily discernible than daily samples .
The composite samples were normally collected over a 6-hr. period , but an occasional 24-hr. composite was made .
Examination of the operations of the shopping center permitted correlation of the 6-hr. composite samples with 24-hr. operations .
The data indicated that the organic load during the 6-hr. composites was essentially 50 per cent of the 24-hr. organic load .
Grab samples were collected from the existing oxidation pond to determine its operating conditions .
Efforts were made to take the grab samples at random periods so that the mass of data could be treated as a 6-hr. composite sample .
A single 24-hr. composite sample indicated that the sewage flow pattern and characteristics were typical .
Pilot plant operations
The BOD of the influent to the pilot plant varied between 110 and 710 mg/l with an average of 350 Aj .
This was equivalent to 240 mg/lBOD on a 24-hr. basis .
The BOD of the raw sewage was typical of domestic sewage from a subdivision .
The BOD in the effluent averaged 58 mg/l , a 76-per-cent reduction over the 24-hr. period .
Examination of the data in Table 1 , shows that a few samples contributed to raising the effluent Aj .
The periods of high effluent BOD occurred during cold periods when operational problems with the aerator resulted .
Ice caused the aerator to overload , straining the drive belts .
The slippage of the drive belts caused the aerator to slow down and reduce oxygen transfer as well as the mixing of the raw sewage .
The organic loading on the unit averaged 32 lb of BOD/day or about 2 lb BOD/day/1,000 cu ft aeration capacity .
Needless to say , the organic load was very low on a volumetric basis , but was 270 lb BOD/day/acre on a surface loading basis .
It seems that the aerated lagoon was a very heavily loaded oxidation pond or a lightly loaded activated sludge system .
The flow rate remained relatively constant during the winter months as shown in Table 1 .
With the spring rains the flow rose rapidly due to infiltration in open sewers .
As construction progresses , the volume of storm drainage will be sharply reduced .
The retention period in the aerated lagoon ranged from 9.8 to 2.6 days , averaging 6.4 days .
The large amount of vegetable grindings from the grocery store in the shopping center created a suspended solids problem .
The vegetables were not readily metabolized by the bacteria in the aeration unit and tended to float on the surface .
A skimming device at the effluent weir prevented loss of most of these light solids .
The average volatile suspended solids in the effluent was 75 mg/l while MLSS averaged 170 mg/l volatile suspended solids .
The average sludge age based on displacement of solids was calculated to be 14.5 days .
The oxygen uptake rate in the mixed liquor averaged 0.8 mg/l/hr during the first four months of this study .
Variations in aerator speeds during the latter two months of this study caused increased mixing and increased oxygen demand .
The increase in oxygen uptake rates from 1.2 to 2.6 mg/l/hr which followed an increase in rotor speed was believed to be related to resuspension of solids which had settled at the lower rotor speeds .
It appeared that most of the mixed liquor suspended solids were active microbial solids with the heavier , less active solids settling out .
The suspended solids discharged in the effluent were found to be the major source of the Aj .
Removal of the suspended solids by a membrane filter yielded an average effluent containing only 20 mg/l Aj .
The BOD in the drainage ditch receiving the pilot plant effluent averaged 12 Aj .
This low BOD was due to removal of the excess suspended solids by sedimentation since the only dilution was surface runoff which was very low during this study .
Routine microscopic examinations were made of the mixed liquor as indicated by McKinney and Gram for the various types of protozoa .
It was found that the aerated lagoon was an activated sludge system rather than an oxidation pond .
At no time were algae found in the mixed liquor .
The bacteria formed typical activated sludge floc .
The floc particles were all small as the heavier floc settled out .
Initially , the flagellated protozoa predominated , but they soon gave way to the free swimming ciliated protozoa .
As the temperature decreased , the number of free swimming ciliated protozoa decreased .
Very little protozoa activity existed below 40-degrees-F .
When the temperature reached 32-degrees-F all protozoan activity ceased ; ;
but as the temperature rose , the numbers of protozoa increased rapidly .
Only once were stalked ciliates found in the mixed liquor .
The predomination of free swimming ciliated protozoa is indicative of a high bacterial population .
One of the important aspects of this study was to determine the oxygen transfer relationships of the mechanical aerator .
Routine determinations were made for dissolved oxygen in the mixed liquor and for oxygen uptake rates .
The data given in Table 2 , show the routine operation of the aerator .
The dissolved oxygen in the aeration unit was consistently high until January 29 , 1961 .
An extended cold spell caused ice to build up on the aerator which was mounted on a floating platform and caused the entire platform to sink lower in the water .
The added resistance to the rotor damaged the drive belts and reduced the oxygen transfer capacity .
It was approximately one month before the belt problem was noticed and corrected , but at no time was there a deficiency of dissolved oxygen .
A series of eight special tests were conducted at different rotor speeds to determine the oxygen transfer rate .
Five of the tests were conducted with a polyethylene cover to simulate an ice cover .
The rate of oxygen transfer at 1.0-mg. / l. dissolved oxygen concentration and 10-degrees-C for various rotor speeds is given in Table 3 .
The maximum rate of oxygen transfer at 1.0 mg/l dissolved oxygen was calculated as 220 lb/day at a maximum rate of 9.3 Aj .
The actual power requirements indicated 2 lb. oxygen transfer / hr. / hp. .
The polyethylene cover reduced the oxygen transfer rate by 10 per cent , indicating that the maximum oxygen transfer is at the rotor rather than through the surface .
During this study septic conditions developed in the oxidation pond in the spring when the ice melted .
Shortly after this study ended septic conditions resulted which required the addition of sodium nitrate .
The location of the oxidation pond in a high-value residential area makes odor nuisances a sensitive problem for the developer .
The organic concentration in the influent raw sewage ranged from 160 to 270 mg/l of BOD with an average of 230 Aj .
The BOD data are given in Table 4 .
A single 24-hr. composite sample had a BOD of 260 mg/l , indicating a typical domestic sewage .
The daily sewage volume to the oxidation pond averaged 147,000 gpd , giving a retention period of 42 days .
The organic loading on the pond was slightly under 60 lb. BOD / day / acre .
The effluent BOD averaged 34 mg/l , a little lower than that of the study at Fayette indicated for a loading of 60 lb. Aj .
The BOD of the effluent ranged from a minimum of 13 to a maximum of 47 mg. / l. .
Microscopic examination of the effluent showed that minimum BOD occurred when the algae began to decrease with cold weather .
When the algae began to build up again , the effluent BOD rose .
During the two weeks when the algae disappeared from the effluent BOD's in the effluent were 18 and 16 Aj .