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Wastewater Treatment System

A few weeks ago I got a discussion about wastewater treatment system in our proposed plant. I discussed with one of my colleague. He said, “Let the nature works! Let it process the wastewater by itself”. It sounds weird at the first time. But he’s right and he’s a bit wrong.

He’s right because the natural water in streams, rivers, lakes, and reservoir have a natural waste assimilative capacity to remove solids, organic matter, even toxic chemicals in wastewater. He’s a bit wrong because the process takes a really long time. That’s why we need wastewater treatment system to make it quick (read more in Water and Wastewater Calculation Manual).

Wastewater treatment are classified into three major categories:

  • Primary treatment (physical process)
  • Secondary treatment (biological process)
  • Tertiary treatment (combination of physical, chemical and biological process)

Each process should include preliminary process, disinfection, and sludge management (treatment and disposal).

Preliminary System

Preliminary systems are designed to physically remove or cut up the larger suspended materials and to remove excessive amounts of oil and grease. The processes does not improve the quality of wastewater. The purpose is to protect pumping equipment and the subsequent treatment units. The systems consist of racks and screens, comminuting devices, grit chambers, preaeration tanks, and possibly chlorination.

Primary Treatment

The objectives of primary treatment is to reduce flow velocity of wastewater sufficiently to permit suspended solid to settle. Suspended solid materials are removed by skimming, thus a primary treatment device may be called a settling tank (basin).

The physical process of settling removes 50%-70% of total suspended solid from wastewater. BOD removal efficiency by primary system is 25% to 35%.

Secondary Treatment

The purpose of secondary treatment is to remove the soluble and colloidal organic matter which remains after primary treatment. Colloidal organic matter should be removed before discharging to receiving waters to avoid interfering with subsequent downstream users.

Secondary treatment or biological process uses controlled natural process in which a very large number of microorganism consume soluble and colloidal organic matter from the wastewater.

Secondary treatment may be categorized into two groups: attached and suspended growth processes.

  1. Attached (film) processes are trickling filters, rotating biologic contactors, and intermittent sand filters
  2. Suspended growth processes include activated sludge, aerobic and anaerobic digestors, anaerobic filters, stabilization ponds, and aerated lagoon

Secondary treatment processes may remove more than 85% BOD. However they are not effective to remove microorganism. Densities of microorganism is reduced by disinfection.



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