12.3 Small Wastewater Systems, Septage Handling and Odor Control
Key Takeaways
The SWWS exam devotes 47 percent of its items to monitoring and adjusting treatment processes, 18 percent to security, safety and administration, 15 percent to equipment maintenance, 8 percent to lab work, and 6 percent each to influent evaluation and equipment operation.
Small systems often use septic tank effluent pump or gravity collection (STEP or STEG), recirculating gravel or sand filters, lagoons and package extended aeration plants.
Septage is many times stronger than sewage, so receiving stations screen it, meter and manifest it, and feed it slowly to avoid shock loads.
Before land application in Oregon, septic tank pumpings must be screened and held at pH 12 for at least 30 minutes; holding-tank blends need pH 12 for 2 hours and 11.5 after 22 more hours.
Odor control can prevent sulfide (oxygen or nitrate), precipitate it (iron salts), keep it in solution (magnesium hydroxide), oxidize it (hydrogen peroxide or hypochlorite), or treat foul air (biofilters, scrubbers, carbon).
The Small Wastewater System Certificate
A DEQ Small Wastewater System (SWWS) is a Class I treatment type with fewer than 500 design population or fewer than 150 connections. The ABC/WPI Small Wastewater System exam weights its content as follows:
| Job area | Share of exam |
|---|---|
| Evaluate incoming wastestream and sidestream characteristics | 6% |
| Monitor, evaluate and adjust treatment processes | 47% |
| Collect samples and perform and interpret lab analyses | 8% |
| Evaluate and maintain equipment | 15% |
| Operate equipment | 6% |
| Perform security, safety and administrative procedures | 18% |
Treatment topics include preliminary treatment, clarifiers, activated sludge, fixed-film reactors, aerated and non-aerated ponds, chemical addition, chlorination and dechlorination, effluent discharge and reuse, solids handling, odor control and septage.
Small-System Technologies
| Technology | How it works | Operator focus |
|---|---|---|
| STEP / STEG collection | Each home keeps a septic tank; effluent is pumped (STEP) or flows by gravity (STEG) through small-diameter mains | Tank pumping schedules, effluent filters, pump vaults and alarms; sulfide and odor at discharge points |
| Recirculating gravel or sand filters | Septic tank effluent is dosed repeatedly over a media bed; part recirculates, part discharges | Dosing timer settings, distribution laterals, ponding, media replacement |
| Package extended aeration plants | Small activated sludge units with long sludge age | DO, sludge wasting, clarifier skimming, blower maintenance |
| Lagoons | Covered in the lagoon section | Depth, dikes, algae, sludge |
| Subsurface disposal and drip | Effluent dispersed in soil under a WPCF permit | Dosing, flushing laterals, monitoring wells |
DEQ may require certified operators for NPDES or WPCF-permitted subsurface systems over 2,500 gallons per day, depending on complexity (OAR 340-049-0015(4)).
Septage Receiving
Septage, the contents pumped from septic tanks, holding tanks, vault and chemical toilets, is far stronger than raw sewage. It is high in solids, grease, grit, ammonia and odors, and it can contain toxics.
Good receiving practice:
- Control access: only permitted or known haulers, with a manifest stating the source and volume. Oregon sewage disposal service businesses are licensed by DEQ.
- Screen and remove grit: rags, plastics and hair are removed before the septage joins plant flow.
- Meter and sample: measure each load; check pH and odor; refuse suspicious loads.
- Equalize and bleed in: store septage and feed it slowly, often at night or at low flow, so it does not shock the biological process or overload solids handling.
- Account for load: one truckload can add as much BOD as hundreds of homes do in a day, so track septage in process control.
Oregon rules for land application of septage (OAR 340-050-0026(5))
- Screen septage first, and dispose of screenings at a permitted landfill.
- Septic tank pumpings: add and mix alkaline material until the pH is 12 or higher, and hold it at pH 12 or higher for at least 30 minutes without adding more.
- Holding tank, chemical toilet and vault toilet pumpings: first blend at least 3 gallons of septic tank pumpings per gallon. Raise the blend to pH 12 or higher for at least 2 hours; after 22 more hours the pH must still be at least 11.5.
- Land application also needs DEQ site authorization and a septage management plan.
Odor Control
Most wastewater odors come from hydrogen sulfide (H2S), which people can smell at well under 1 part per billion. Other culprits are mercaptans, amines and volatile fatty acids. Sulfide forms when sulfate-reducing bacteria work in anaerobic slime layers, especially in long force mains, wet wells with long detention, and septic sludge.
| Strategy | Examples | Mechanism |
|---|---|---|
| Prevent sulfide formation | Air or oxygen injection; calcium nitrate addition | Keeps conditions aerobic or anoxic, so bacteria use oxygen or nitrate instead of sulfate |
| Precipitate sulfide | Ferrous or ferric chloride, ferrous sulfate | Iron sulfide precipitates in the liquid |
| Keep sulfide in solution | Magnesium hydroxide or caustic to raise pH | At higher pH sulfide stays as HS- instead of escaping as H2S gas |
| Oxidize sulfide | Hydrogen peroxide, sodium hypochlorite, permanganate | Destroys existing sulfide quickly |
| Treat foul air | Biofilters, biotrickling filters, chemical wet scrubbers (caustic and hypochlorite), activated carbon | Removes H2S from ventilated air |
| Contain | Covers, sealed channels, negative ventilation | Keeps odors from escaping |
Biofilters pass foul air through moist organic or engineered media where bacteria oxidize H2S. Empty-bed contact times of about 30 to 60 seconds are common, and the media must stay moist and be watered and replaced as it compacts or acidifies. Wet scrubbers need correct pH and oxidant dose, and scale control.
Keep an odor complaint log with the time, location, weather and plant conditions. Patterns point to the source, and documented, prompt responses build public trust.
Routine Records for a Small Plant
Even a one-operator lagoon or recirculating gravel filter needs a simple, consistent log:
- daily or visit-by-visit flow readings, pump run times and alarm events;
- visual checks: color, odor, scum, solids carryover, berm and fence condition;
- field tests and lab sample dates, times, locations and results, plus chain-of-custody copies;
- maintenance done, chemicals used and septage or sludge quantities hauled;
- any overflow, bypass or permit exceedance, with the date and time it was reported.
Good records support the monthly DMR, show the work during DEQ inspections, and count as documented experience when the operator later applies for a higher certificate.
On the Small Wastewater System exam, which job area carries the largest share of items?
Monitoring, evaluating and adjusting treatment processes
Collecting samples and performing laboratory analyses
Evaluating incoming wastestream characteristics
Performing security, safety and administrative procedures
A hauler wants to land-apply pumpings from residential septic tanks in Oregon. What stabilization does OAR 340-050-0026 require after screening?
Add chlorine to a residual of 50 mg/L and hold it for 6 hours
Raise the pH to 12 or higher and hold it for at least 30 minutes
No treatment is required for septage from residential tanks
Aerate the septage for at least 24 hours before it is spread
A utility adds calcium nitrate at the upstream end of a long force main. How does this reduce hydrogen sulfide odors?
Nitrate oxidizes the ammonia in the sewage directly to nitrogen gas
Nitrate precipitates the dissolved sulfide as an insoluble solid
Nitrate raises the wastewater pH above 11, keeping sulfide dissolved
Bacteria use nitrate instead of sulfate, so sulfide is not produced
Sections you finish are checked off in the contents.