Biological Wastewater Treatment Methods / Secondary Treatment
1. Objectives of Secondary Treatment of waste water
The main objective of secondary treatment: To remove most of the fine suspended and dissolved degradable organic matter that remains after primary treatment, so that the effluent may be rendered suitable for discharge. Conventional secondary treatment can reduce the BOD's to below 20mg/l and Suspended Solids to below 30mg/l which is acceptable in many cases.
The second objective of secondary treatment: The reduction of ammonia toxicity and nitrification oxygen demand in the stream. This is achieved by oxidation of most of the ammonia to nitrate during treatment (nitrification).
Means the oxidation of ammonia to nitrate. Nitrification is possible with aerobic biological processes. If they are operated at low organic load rates-hence the units must be large than those which would be required for oxidation of carbonaceous matter alone.
Biological Wastewater Treatment Method
It comprises of the following sub processes:
Aerobic Biological Processes are those where sufficed amount of dissolved oxygen is required into the wastewater to sustain aerobic action, as one of the major polluting effects of wastewater on streams results form the depletion of dissolved oxygen by the action of aerobic organisms in degrading the organic content of the waste. Practical aerobic biological treatment processes seek to to this, within the constraints of available land area and economic resources available to construct and operate treatment works.
Anaerobic Biological processes are those where micro-organisms oxidize organic matter in the completed absence of dissolved oxygen. The micro-organisms take oxygen form inorganic salts which contain bound oxygen (Nitrate NO3, Sulphate So42-, Phosphate PO42-). This mode of operation is termed as anaerobic processes. Sufficiently fore dissolved oxygen is either physically difficult or economically impracticable to transfer into the wastewater to sustain aerobic action to biodegrade strong organic wastes.
Aerobic Biological Treatment Processes
There are five types of aerobic biological treatment processes used to treat municipal sewage:
Biological Treatment systems
The trickling filter is like a circular well having depth up to 2 meter filled with granular media like stone, plastic sheets and redwood, slag, slate.
The first tricking filter was placed in operation in England in 1893. The concept of a tricking filter grew from the contact frets which were water tight basins filled with broken stones. The limitation of the contact filters included a relatively:
Tricking filters have been a popular biological treatment processes. The widely used design of trickling filters that can last for many years is:
Design diameter of Rock filters = 60m (2007t) and for Rock size Design diameter = 25 to 100mm