Secondary wastewater treatment

Effluent form primary treatment which consists of dissolved organic and inorganic substances such as glucose, nutrient, protein and ashes will be treated in secondary treatment. It employs biological treatment where microorganisms consume the organics in the waste water and convert into carbon dioxide, water and energy for reproduction.

Microorganisms used in secondary treatment play important role in designing the biological system due to the factors such as the kinetics of metabolism and controlled environment inside the reactor for the growth. In this waste water treatment, type of microorganism used is heterothrops which consumed organic carbon as nutrient source with provided aeration (schuler and kargi).

There are 2 types of continuous reactor system may be used in biological treatment which are suspended-culture system and attached culture system. In suspended-culture system, either single or clustered cells are maintained in the suspension rich in nutrient and proper mixing is provided. Attached-culture system microorganisms are attached onto the inert surfaces and waste water rich in nutrient passing through the microbial biofilm (howard). To choose the suitable reactor system consideration is made based on characteristics between activated sludge, lagoon and trickling filter.

1. Operational characteristics (Source: Rich, L. G.; 1980 )

System Resistance to shock loads of organics or toxics

Sensitivity to intermittent operations

Degree of skill needed

Activated sludge Minimum Maximum MaximumLagoon Maximum Minimum MinimumTrickling Filter Moderate Moderate Moderate

2. Cost consideration

System Land needed Initial Cost Operating costActivated sludge Minimum Maximum MaximumLagoon Maximum Minimum MinimumTrickling Filter Moderate Moderate Moderate

Based on the characteristics above, trickling filter is chose as the reactor for the secondary treatment due to its moderate in operation and cost. Trickling filters

Rich, L. G.; 1980. Low Maintenance, Mechanically Simple Wastewater Treatment Systems, McGraw-Hill Book Co., New York; U.S.A.