The aerobic units discussed here are for systems receiving less than 2000 gallons per day.
 Procedures & Schedules |
Operational Problems |
| Personnel & Contracted Work |
Operation & Maintenance Requirements |
| Parts, Supplies and Equipment |
Comparison to Past Work |
| Example Annual Costs Per Home |
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1. Inspection: every 1 to 2 months
a) Visually check aeration and mixing, inspect diffusers, clean or replace as required. Record.
b) Perform half hour cylinder settlement test on mixed liquor. Record.
c) Observe motor-blower operation. Clean/replace air filter and oil as necessary. Clear air intake of weeds/other obstructions. Record.
d) Check sludge and scum levels. Remove floating solids as necessary. Brush down tank. Record.
e) Check trash trap for accumulated solids.
f) Manually operate and check alarm. Record.
2. Unscheduled service calls (infrequent, average 1 per 5 years)
3. Pump sludge as required (assume every 12 months).
4. Homeowner education.
5. For surface discharge, monitor and record biochemical oxygen demand (BOD5), suspended solids (SS), and fecal coliforms (FC) quarterly.
Indiana does not allow surface discharge from individual residences from any on-site system.
Personnel: inspection and service person with mechanical, chemical, or biological background, ability to work without supervision, will need specific training on the various units utilized
Solids Pumping:
Chemical analysis for surface discharge (BOD5, SS, FC):
As surface discharge is not legal in Indiana, chemical analysis is not necessary.
Parts and Supplies: air filter, compressor vanes, compressor, pump, lubricants, etc. $75/plant/yr.
Equipment:
| 1000 ml graduated cylinder | $14/2 yrs |
| garden hose | $20/2 yrs |
| gloves | $6/1 yr. |
| thermometer | $10/1 yr. |
| grease gun | $13/5 yrs |
| bucket | $ 9/2 yrs |
| brushes | $15/1 yr. |
| skimmer net | $15/1 yr. |
| Total: | $70/1 yr. |
Power: 8 kwh/day, $0.07/kwh
| Labor: | 20 man-hrs * $10/man-hr = | $200.00 |
| Solids Pumping: | 1 pumpings * $60/pumping = | $ 60.00 |
| Supplies and Parts: | $ 75.00 | |
| Equipment: | $70 / 30 homes = | $ 2.33 |
| Power: | 8 kwh/day * 365.00 days * $0.07/kwh = | $204.40 |
| Vehicle: | 6.20 services * 4 average miles/service * $0.38/mile = | $ 9.42 |
| Total | (First Quarter 1982): | $551.16 |
| Updated Total | Total | OMR | 1982 OMR | |
| $551.16 | * 3.49 | / 3.44 | $559.17 |
In at least one area of Indiana, it is possible to obtain an annual service contract for between $135-150/yr. Look for a system that includes a flow equalization component. Also, energy efficient models may now be available.
These extended aeration units can be used in surface discharge applications and where other systems will not work. They are mechanically complex and very expensive to operate. Typically, they service a single home.
Very little data is available for units in the 500 to 2,000 gpd range however, it is reasonable to assume that costs vary linearly with flow in this limited range (two to five homes).
Proper aeration and mixing is critical to the performance of these systems. Air diffusers sometimes clog. The mechanical parts demand regular attention to seals, lubrication, filters, and controls. Sludge and scum accumulate as in septic tanks.
| Observation | Cause | Remedy |
| Excessive local turbulence in aeration tank | Diffuser plugging Pipe breakage Excessive aeration | Remove and clean Replace as required Throttle blower |
| White thick billowy foam on aeration tank | Insufficient MLSS* | Avoid wasting solids |
| Thick scummy dark tan foam on aeration tank | High MLSS* | Waste solids |
| Dark brown/black foam and mixed liquor in aeration tank. | Anaerobic conditions Aerator failure | Check aeration system, aeration tank. D.O. |
| Billowing sludge washout in clarifier | Hydraulic or solids overload Bulking sludge | Waste sludge; check flow to unit See reference (37) |
| Clumps of rising sludge in clarifier | Denitrification | Increase sludge return rate to decrease sludge retention time |
| Septic conditions in clarifier | Increase return rate | |
| Fine dispersed floc over weir, turbid effluent | Turbulence in aeration tank | Reduce power input |
| Sludge age too high | Waste sludge |
*Mixed liquor suspended solids
Often manufacturers recommendations for these systems are so much less conservative than those of other published sources as to be unreconcilable. Most sources of actual experience tend to be somewhat less conservative than the EPA Design Manual (1980c). Recommendations given here lean toward the conservative butpractical.
The inspections detailed here may be performed every one to two months, with 20 man-hours per year allowed for all inspection and service work (EPA, 1980c; Abney, 1980; Fancy, 1980; McEnterfer, 1982; Arizona Anti-septic Systems, 1981). The half hour cylinder settlement test consists of observing 1,000 ml ofmixed liquor in a graduated cylinder for one half hour, andrecording the level of settled sludge at specified times. Thiswill indicate the quality of floc, aeration problems, etc.
With a regular maintenance program, emergency serving should average less than one visit per home in five years (Fancy, 1980; Cruce, 1981). The service person qualifications listed are given by Fancy (1980).
Pumping of accumulated solids is necessary periodically although sometimes sludge may be wasted to a septic tank for storage. Here again, sources varied widely, from one pumping in eight months, to five years, to never (EPA, 1980c, Abney, 1980; Boyle and Otis, 1979; Arizona Antiseptic Systems, 1981; Jet Inc., 1980; Environmental Dynamics Inc., 1981; McEnterfer, 1981).
If the unit discharges to surface water, effluent monitoring is legally required, though frequency may vary locally. An education program promoting homeowner understanding of the systems would also be a valuable management function.
There are many other requirements for these individual aerobic units.
Replacement parts commonly cost $75 per year (McEnterfer, 1980; Fancy, 1980). Reported power consumption varies from 2 to 10 kwh per day (McEnterfer, 1981; EPA, 1980c; National Utility Contractors Assn., 1979; Boyle and Otis, 1979; Otis et al., 1977; Abney, 1980).
The annual expense per home for a group of 30 homes has been calculated, assuming four vehicle miles for the average service as was done for septic tanks.
Fancy (1980) has found an average annual cost per home of $130 (to a maximum $330) for labor and $64 for parts (when updated with the OMR index). This was for an area with a high density of aerobic units.
Using the example annual expense per home calculations, $200 and $75, respectively, are obtained.
This margin of safety allows for cases where fewer homes utilize aerobic unit, making labor less efficient.
