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Weir trough

Primary Treatment of Wastewater


Aims of primary treatment of wastewater is to separate suspended solids and greases from wastewater. What does primary treatment of wastewater remove? Sedimentation remove both organic and inorganic suspended solids. In addition, skimming, as part of sedimentation tank, remove grease, oil, scum, and floating solids.

Primary treatment of wastewater includes primary sedimentation (also called clarification), microscreen, Imhoff tanks. In most facilities, primary treatment is used as preliminary step ahead of biological treatment.

Primary treatment of wastewater processes
Primary treatment of wastewater processes

Primary Sedimentation

Sedimentation tank hold wastewater for several hours allowing the particle to settle to the bottom and the greases to float atop. Sedimentation can be accompanied with or without chemical addition. During sedimentation process, the settling of particles become larger, as a result of particle agglomeration or flocculation (flocculent sedimentation). Stirring, adding chemicals, and biological agents can stimulate sedimentation process.

Schematic of circular primary sedimentation tank
Schematic of circular primary sedimentation tank [3]
Sedimentation tanks are designed to operate continuously. They are usually circular or rectangular and have hopper for sludge collection. Most sedimentation tanks are constructed with gently sloped bottoms and have sludge hoppers with relatively steep sides. In small installation, there are non-mechanized settling tanks (made of concrete). Then the sludge moves to hoppers by gravity, where it is removed.

Settling reservoir with circular layout with sludge scraper and floating scum scraper
Settling reservoir with circular layout with sludge scraper and floating scum scraper [2]
When used as the only means of treatment (no longer authorized in U.S. and Trust territories), sedimentation tanks provide for removal of settleable solids and much of the floating materials. When used as a preliminary step to biological treatment, their function is to reduce the load of biological treatment units (where its removal is more expensive). Efficiently designed and operated primary sedimentation tanks should remove approximately 25% to 50% of the incoming BOD and 50% to 70% of the total suspended solids (SS).

Factors Affecting Sedimentation Process

The efficiency of sedimentation depends on:

  • The size and the form of the particles. The larger or bigger the particle size, the faster sedimentation occurs.
  • The density of the particles. If the difference between the particle density and the carrier liquid is larger, then the sedimentation process is faster
  • The composition of the suspension.
  • The concentration of the suspension; the larger the concentration, the larger the sedimentation process efficiency.
  • The suitability of particles to flocculation.
  • The temperature, with higher temperatures, the viscosity of the liquids is reduced and thus the particles settle faster.
  • The depth and form of the sedimentation reservoir.
  • The distance that the liquid travels through reservoir.
  • The velocity of the liquid.
  • The influence of the wind of the liquid’s surface.
  • The way in which the liquid flows into the reservoir and is removed.
  • The occurrence of short-circuits, resulting from the fact that the liquid has already, however small, a difference in density.

Disturbance Through Turbulence Flow

In reality, sedimentation is influenced by turbulence in the tank. When turbulence occurs, a few particles will swirl up and most likely to be removed with the effluent and a few particles will reach the sludge zone earlier. Turbulence unfavorably influences sedimentation efficiency.

The rate of flow is characterized by Reynolds Number, that should be lower than 2000 to have laminar flow.

Disturbance by turbulence
Disturbance by turbulence [2]

Disturbance Through Short-Circuit Flows

When short-circuit flows occur in a sedimentation tank, dead zones and eddies occur, the average residence time is shorter than in theory. Eddies are circular movement of water, counter to main current, causing a small whirlpool. Short-circuit flows occurs when there is a difference in the liquid density in the reservoir and that which is delivered. Short-circuit flows are illustrated in the figure below.

Disturbance by short-circuit flows
Disturbance by short-circuit flows [2]
To limit the short circuit flows as much as possible, a good distribution of the incoming and outgoing liquid is necessary as well as a stable flow pattern. A stable flow is the recovery of the original state of flow after occurrence of a disturbance. The stability of a flow is higher as the ratio between the force of inertia and gravity is bigger. As the flow becomes more stable, the distribution of velocity over the cross-section is more constant and the flow will, after the occurrence of a disturbance, recover faster returning to its original state. Froude number reflect ratio between the force of inertia and gravity. The number should be higher than 10-5.

Design Parameters – Surface Loading Rate

Sedimentation tanks are usually designed for the average daily flow or daily flow equivalent to the peak hourly flow that requires the largest surface area. Tables below can be used as guidance to select the correct surface loading rate. All tank piping, channels, inlets, outlets and weirs should be designed to accommodate peak flows. If specific peak flows are not documented, the peak flow must be based on the average daily flow or average hourly flow increased by appropriate peaking factor. The peak factor is between 2.5 to 4 [6].

Surface loading rates for primary sedimentation tank
Surface loading rates for primary sedimentation tank [1]
Surface loading rates for primary sedimentation tank in SI unit
Surface loading rates for primary sedimentation tank in SI unit

Other source mentioned that in general recommended maximum surface loading rate is 1.5 – 2.5 m3/(m2.h). If this maximum load has little or no effect on the influent flow pattern, then higher values, up to 4 m3/(m2.h) are used [2].

Design Parameters – Detention Time

Several sources mentioned different detention times. Detention time is commonly specified as 2.5 hours for primary tanks serving all types of plants except when preceding an activated sludge system, where detention time is specified at 1.5 hours [1]. Other source mentioned that wastewater velocity to sedimentation tank should be reduced to allow detention time between 2 to 4 hours [4]. Selection of optimum detention time will depend on the tank depth and the overflow rate.

Design Parameters – Weir Overflow Rate

The wastewater discharge from a sedimentation tank usually takes place through one or more effluent weir trough. When leaving the tank, the liquid accelerates. In order not to danger the stability of the flow in the tank, the discharge must be evenly separated and the so-called weir overflow rate may not exceed certain values.

Weir trough
Weir trough [5]
The weir-overflow rate is the discharge of the effluent (m3) per unit (of length of the effluent weir troughs/(m) per unit of time (h)). If the wire overflow rate is too high, light material can be discharged with overflow water from tank. As a maximum acceptable value of the weir overflow rate of the primary settling tanks is kept at 10 to 15 m3/(m.h) [2]. Other source mentioned different values. Maximum acceptable weir overflow rate is a function of wastewater design flow [1].

The weir overflow rate should not exceed 5000 gallons per day per lineal foot (8.4 m3/(m.h)) for plants designed for less than 0.1 million gallons per day (15.7 m3/h), or 10,000 gallons per day per lineal foot  (16.8 m3/(m.h)) for plants designed between 0.1 and 1.0 million gallons per day (15.7 – 157 m3/h). Weir overflow rate for flows of more than 1.0 million gallons per day (157 m3/h) may be higher, but must not exceed 12,000 gallons per day per lineal foot (20.3 m3/(m.h)). When pumping is necessary, the pump capacity will be related to tank design to avoid excessive weir.

Sedimentation Design Features

It is important to design inlets to sedimentation tanks so that to dissipate the inlet velocity, to distribute the flow uniformly, and to prevent short-circuit flows.

Table below shows guidelines to design the depth of settling tanks.

Settling tank depths
Settling tank depths [1]
Settling tank depths in SI unit
Settling tank depths in SI unit

Other source mentioned that optimal depth is between 1.5 m for small tanks and 2.5 m for large tanks. For rectangular tanks, the ratio of depth/length should be approximately 1:20; the width/length ratio should be minimum 1:4 and preferably 1:5 to 1:6.

Circular Tanks (Round Tanks)

Minimum diameter of circular tanks is 20 m and their maximum diameter is 60 m. Optimal diameter is between 30-40 m [2]. With smaller tank with diameter less than 30 m, the efficiency will be lower due to interference by influent flow and discharge.

Recommended depth of circular tanks is 1.5 to 2.5 m. With bottom slope 1:10 to 1:12.

Rectangular Tanks

Maximum length of rectangular tanks is 90 m. Optimal length is between 30-50 m [2]. Recommended width is 5-12 m, and its depth is 1.5-2.5 m. With bottom slope 1:10 to 1:12.

Collection and Removal of Scum and Sludge

We can choose to remove the scum manually or automatically. After scum removal, we shall ensure that the scum is discharge to separate well or sump. After that, scum can be either sent to the digester or disposed of separately.

Average velocity of sludge scraper should be sufficiently low so that it will not cause churning up previously settled sludge. Maximum allowable peripheral velocity for the sludge scraper on circular tanks is 0.06 to 0.07 m/s, and for a velocity for rectangular tanks approximately 0.03 m/s.

The discharge of the settled sludge usually takes place in a continuous or semi-continuous way. It started with channel or funnels the sludge. And then, sludge is pumping regularly into sludge thickeners. We must avoid build up the sludge in primary tank.

References:

  1. “Joint Departments of the Army and Air Force USA, Technical Manual TM 5-814-3/AFM 88-11, Volume 3, Domestic Wastewwater Treatment, 31 August 1988.
  2. Wastewater Treatment, Delft University of Technology
  3. Marcos von Sperling, 2007. Wastewater Characteristics, Treatment and Disposal. Iwa Publishing, New Delhi
  4. Introduction to Wastewater Treatment
  5. Trough, Wires, and Bafles
  6. Unified Facilities Criteria (UFC) Domestic Water Treatment

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