Wastewater Treatment Planning - Part 1

wastewater treatment plant (WWTP) is a physical plant where various physical, biological or chemical processes are used to change the properties of the wastewater (e.g. by removing harmful substances) in order to turn it into a type of fresh water (effluent) that can be safely discharged into the environment or that is usable for a certain reuse purpose.  The treatment of wastewater belongs to the over-arching field of sanitation, just like the management of human waste, solid waste, sewage treatmen, stormwater and drinking water treatment By-products from wastewater treatment plants, such as screenings grit chamber, sludge unit, odor removal, gases system, and other  unit. A wastewater treatment plant generally requires electrical energy to function, except for certain types of constructed wetlands but they can also produce energy  with anaerobic processes in the form of biogas. There are many kind of wastewater treatment unit which is based on the characteristic of the wastewater. The characteristic of wastewater are classified into physical, biological, and chemical properties. In planning a wastewater treatment system, there are some steps that we should do, they are feasibility study (FS), preliminary sizing (PS), detail engineering design (DED), tender, construction, comissioning, operational and maintenance. In this case, we will only focus on preliminary sizing (PS) and detail engineering design (DED).

 

Preliminary sizing (PS) is the step of sizing dimension of WWTP unit generally. WWTP is built by conducting some treatment units. All of the units should be sizing in order to know the area which is needed to support the WWTP. In preliminary sizing, we calculate the length, area, and the depth of each unit. Steps of preliminary sizing are:

 

1. Determine the design period of WWTP

Design period of WWTP is the lenght of time of WWTP to operate the system. Design period is determine the time of WWTP reach the maximum capacity. This period is determined from the first planning time until final year (full capacity). Design period is influenced by the growth of people, environment, and fund. There are 2 types of design period, initial years and design years. Initial years is the time to finish the construction till the units are already to operate. Design years is the time of WWTP reach the maximum capacity. Generally, design period of WWTP is for 10-40 years.

 

2. Determine the flowchart of WWTP process

Flowchart is determining the systems of WWTP and show what treatment units are used. WWTP units can be classified into physical, chemical, and biological process. Determine of WWTP units is based on the engineer's experiences, local rule, provision of treatment unit, charge of investment and operational. A WWTP flowchart is shown below.

 

3. Determine the design criteria of each unit

Design criteria is general requirement in design of an unit. Design criteria sets the range of unit properties so that it can be design appropriate.  For example this is the design criteria of treatment units.

Bar Screen

- Bar velocity (Vbar)  : 0,3,6 m/s

- Bar width (Wbar)    : 4,0 - 8,0 m

- Bar depth (Dbar)     : 25 - 50 mm

- Distance between bar : 25 - 75 mm

- Vertical slope : 45 - 60 degree

- Maximum Headloss : 800 mm

- Form factor:

1. Sharp-edge rectangular   : 2,42

2. Rectangular with semicircular upstream face   : 1,83

3. Rectangular with semicircular upstream and downstream face   : 1,67

4. Circular   : 1,79

5. Tear shape   : 0,76

Collection Pit

- Retention time (td)   <= 10 minutes

Screw Pump

-Diameter   : 0,3 - 3 m

-Capacity   : 0,01 - 3,2 m3/s

-Slope        : 30-38 degree

-Maximum headloss   : 9 m

-Motor velocity   : 30 - 50 rpm

Grit Chamber

- Retention time (td)   <= 45-90 second

- Horizontal velocity   : 0,25-0,4 m/s

- Settle velocity   : 1,0 - 1,3 m/minute

- Length of channel : 10-20 m

Primary Sedimentation

Rectangular

- Depth : 3 - 4,9 m

- Length   : 15-90 m

- Width   : 3 - 24 m

- Flight speed   : 0,6 - 1,2 m/minutes

Circular

- Depth : 3 - 4,9 m

- Diameter   : 3 - 60 m

- Slope  : 0,75 - 2,0 in/ft

- Flight speed   : 0,02 - 0,05 m/minutes

Aeration Tank

- Depth : 3 - 5 m

- Freeboard  : 0,3 - 0,6 m

- Width:Length   : 1:1 - 2,2:1

- Width   : 3,0 - 11 m

Secondary Clarifier

-Organic Flor Rate   : 8,0-16,0 m3/m2.day
-Solid loading :  0,5-5 kg/m2.hour
-Depth : 3,5-5 m
-Settling Zone Depth : 1,5 m

 

Sludge Thickener

- Solid loading  : 40-78 kg/m2.day

- Solid Sludge   :2,0-8,0 %
- Sludge vol. Ratio   : 0,5-20 dry solid

- Sludge blanket   : 0,6-2,4 m
- Slope : 1,4-1,6

Sludge Digester

- Hydraulic Retention Time in 20oC= 40-78 kg/m2.day

- Solid loading= 1,6-4,8 kg volatile solid/m3.day

- Supply of O2 for cell tissue= 2,3 kg O2/kg solid destroyed

- Energy supply of mixing mech aerator= 19,7-39,5 Kw/103 m3

- Diffused air mixing= 0,02-0,04 m3/m3.menit

- DO residue in liquid= 1,0-2,0 mg/l

- Reduction on VSS= 40-50 % 

Sludge Drying Bed 

- Sand thickness   : 23,0-30,0 cm

- Gravel thickness : 20,0-30,0 cm

- Sludge loading rate : 100-300 kg/m2.years

- Bed thickness : 20,0-30,0 cm

- Bed width :  5,0-8,0 m

- Bed length : 6,0-30,0

- Drying time   : 10,0-15,0 hari

- Uniformity coefficient < 4

- Effective size   : 0,3-0,75 mm

- Water velocity in inlet   : 0,75 m/det

- Water velocity in drain   : 0,75 m/det

 

>> To be continued