Home / Waterpedia
Activated sludge

An aerobic wastewater treatment process where microorganisms are grown in a reactor and kept in suspension through mixing and aeration. Wastewater is passed through the reactor where the suspended microorganisms consume and convert the organic matter, and later being separated from the treated wastewater in a settlement process. The process requires a constant recycling of biomass from the settlement tank back to the reactor to maintain the required microbial population for treatment.


The quantitative capacity of water to neutralize an acid; that is, the measure of how much acid can be added to a liquid without causing a significant change in pH. In the water industry, alkalinity is expressed in mg/l of equivalent calcium carbonate. There are three kinds of alkalinity: carbonate, bicarbonate, and hydroxide alkalinity. Total alkalinity is the sum of all three kinds of alkalinity.


Reversing the flow of water back through the filter media to remove entrapped solids.

Batch Process

A treatment process in which a tank or reactor is filled, the water or wastewater is treated , and the tank is emptied. The tank may then be filled and the process repeated. Batch processes are also used to cleanse, stabilize, or condition chemical solutions for use in industrial manufacturing and treatment processes.


BOD is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. The term is widely used as an indication of the organic quality of water.

Brackish Water

Brackish water conntaining between 1.000 and 15,000 ppm of dissolved solids is water that has more salinity than fresh water, but not as much as seawater . It may result from mixing of seawater with fresh water, or it may occur in brackish fossil aquifers.


A strong solution of salt(s) with total dissolved solid s concentrations. The waste solution in both automatic water softeners and reverse osmosis systems.


Chemical Oxygen Demand (COD): A measure of the oxygen required to oxidize all compounds, both organic and inorganic, in water. COD is typically measured in mg/l, which indicates oxygen consumed per liter of solution.The term is widely used as an indication of the organic quality of water.


Removal of salt (sodium chloride) and other minerals from the sea water to make it suitable for human consumption and/or industrial use. The most common desalination methods employ reverse-osmosis.

Deionaziton/ Demineralization

The term is generally used with respect to removal of ions from water. Deionization is commonly achieved by ion exchange process.Deionization is used for water purification for industrial use.


The breaking down of sludge and other waste biologically by microorganisms. Results in byproducts such as methane gas, carbon dioxide, sludge solids and water. Aerobic digestion requires oxygen, anaerobic digestion the absence of oxygen.


The liquid discharged from a wastewater system component.


The process whereby a chemical or other substance is added to wastewater to trap or attract the particulate suspended solids into clusters or clumps of floc or flocculent, wooly looking masses.


Membrane bioreactor is the combination of a membrane process like microfiltration or ultrafiltration with a suspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment.

Primary treatment

First (sometimes the only) stage of wastewater treatment to remove large solids. Often the first stage of settlement is called "primary settlement". Preliminary treatment to remove wastewater constituents that may cause maintenance or operational problems in the system eg, grit removal, screening for trash and debris, oil and grease removal.


Parts Per Million.A common basis for reporting the results of water and wastewater analysis, indicating the number of parts by weight of water or other solvent. In dilute water solutions, one part per million is practically equal to one milligram per liter, which is the preferred unit.


Reverse Osmosis.A process for the removal of dissolved ions from water, in which pressure is used to force the water through a semi-permeable membrane, which will transmit the water but reject most other suspended and dissolved materials. It is called reverse osmosis because mechanical pressure is used to force the water to flow in the direction that is the reverse of natural osmosis.

Secondary treatment

More advanced treatment than primary treatment. Stage two of the process which reduces suspended solids and organic matter. Typically require a biological treatment stage to remove/reduce contaminants. Examples include biological trickling filter plants, oxidation ponds, aerated lagoons and activated sludge plants.


The solid waste material which settles out in the wastewater treatment process, sometimes biosolids. Can be dewatered and reused or disposed.


Total Dissolved Solids is the combined total of all dissolved solids in wastewater, both organic and inorganic and very fine, such as colloidal minerals. Generally particles must be smaller than two micrometers to be considered a dissolved solid. For example, salt dissolved in water is a dissolved solid. Therefore TDS will "survive" screening or other coarse filtration.

Tertiary treatment

Final stage of wastewater treatment, usually to remove nutrients and pathogens eg, additional biological treatment stage, filtration, chemical flocculation and ultra-violet light disinfection.


Total Suspended Solids (TSS) is the measure of combined settleable and non-settleable solids in wastewater. TSS is a measure of the concentration of solids in suspension, which correlate to the amount of treatment required.


A measure of the amount of finely divided suspended matter in water, which causes the scattering and adsorption of light rays. Turbidity is usually reported in arbitrary nephalometric turbidity units (NTU) determined by measurements of light scattering. NTU should not exceed 0.5 in potable water. Turbidity can protect bacteria from sterilization.


Wastewater is "used" water, the water leftover after its use in numerous application such as industrial, agricultural, municipal, domestic and on.


Water Treatment Plant


Waste Water Treatment Plant