How is solids-separation controlled in primary wastewater treatment?

Feb. 1, 2015

Solids —including those suspended, settle-able or floatable — are non-liquid or fluid substances separated from clarified wastewater in primary treatment.

Solids —including those suspended, settle-able or floatable — are non-liquid or fluid substances separated from clarified wastewater in primary treatment.

Suspended solids are those small, solid particles that remain so in water, as a colloid, or due to the motion of the water.

Settle-able solid sare particles that will settle within a reasonable length of time assuming little movement. The weight by volume of settle-able solids is greater than water.

Floatable solids do so because their weight by volume is less than that of water.

Primary wastewater treatments that remove these solids in process containers include primary clarifiers, air-flotation units and cone-shape Imhoff settlers. While effective solids separation supersedes control of pH and flow or other factors, generic controls for each process container are described in the following.   

Primary clarifiers

The primary clarifier’s most important function is removing as much settle-able and suspended material as possible. Unless removed, organic settle-able solids can cause a high demand for oxygen (BOD) in subsequent biological treatment in the plant or receiving waters.

Many factors influence the settling characteristics of a given clarifier. Most common include temperature variation, short circuits, detention time, weir-overflow rate, surface-loading rate and solids loading. Three of these factors are discussed below.

1. Temperature:  In general, as water temperature increases, the settling rate of particles increases; as temperature decreases, so does the settling rate.

2. Short circuits: As wastewater enters the settling tank, it should be evenly dispersed through the entire cross-section of the tank. It should flow at the same velocity in all areas toward the discharge end. If velocity is greater in some sections than others, short-circuiting may occur.

Short-circuiting also may be caused by turbulence or density-layer stratification following from temperature or salinity variation. Temperature layers can cause short-circuiting.

3. Detention Time: Wastewater should remain in the clarifier long enough to allow sufficient solid-particle settling time. If the tank is too small for the flow quantity and the particle settling rate, too many particles will be carried out the effluent of the clarifier.

Air flotation units

The objective of flotation thickening is to separate solids from the liquid phase by matching air bubbles to particles of suspended solids. Four general methods of flotation are common, as follows:

1. Dispersed air flotation where bubbles are generated by mixers or diffused aerators.

2. Biological flotation where gases formed by biological activity are used to float solids.

3. Dissolved air, vacuum flotation where water is aerated at atmospheric pressure and released under a vacuum.

4. Dissolved air, pressure flotation where air is put into solution under pressure and released at atmospheric pressure.

The performance of dissolved air-flotation units depends on; type of sludge, age of the feed sludge, solids and hydraulic loadings, air-to-solids (A/S) ratio, recycle rate, and sludge-blanker depth.

Operational guidelines are affected by the size of dissolved air flotation units.

Performance can be optimized by controlling the air-to-solids ratio, recycle rate and blanket thickness, by properly adjusting these variables. These control variables are affected by solids and hydraulic loading rates. Controls should prevent solids or hydraulic loading from becoming excessive, which would degrade effluent quality and reduce thickened sludge concentrations.

Performance is optimized by conditions such as those described in the following.

Air-to-solids ratio: An optimum ratio should be established and maintained. It is affected by the quantity of air introduced and dissolved into the recycle or waste-stream, which is in turn critical to flotation thickener operation. Enough air has to be added and dissolved to float the sludge solids. The most effective way to do this is introducing air into a pressurized retention tank along with the waste-stream to be thickened, or along with a portion of the thickener effluent stream. Air also can be dissolved in primary or secondary effluent, thus avoiding solids recycling in the dissolved air flotation (DAF) unit.

Mixing the retention tank contents should also be used to increase air put into solution.

Recycle rate and sludge blanket: Both the rate of effluent recycle and the thickness of the sludge blanket are operational controls available to optimize DAF performance.

Note on Imhoff tanks

Imhoff tanks are rarely constructed today. Your plant may incorporate an Imhoff tank only if was constructed many years ago, perhaps in the 1980s. Cone-shape Imhoff settler tanks are unique because the Imhoff tank combines sedimentation and sludge digestion in the same process unit.

The majority of primary industrial wastewater-treatment solids-separation process units operate with clarifiers and flotation devices, which are covered in this article.

Certain general steps related to separating solids during primary wastewater treatment have been described above. However, if you have specific solids separation or other wastewater queries, please submit a question.