introduction to waste water treatment

Introduction

Filtration Spectrum

 what is osmosis?

A semi--‐permeable membrane is a membrane that will allow some atoms or molecules to pass but not others. A simple example is a screen door. It allows air molecules to pass through but not pests or anything larger than the holes in the screen door. Another example is Goretex clothing fabric that contains an extremely thin plastic film into which billions of small pores have been cut. The pores are big enough to let water vapor through, but small enough to prevent liquid water from passing.

Any type of substance moves from area of highest energy or concentration to region of lowest energy or concentration.	Only water or another solvent moves from a region of high energy or concentration to a region of lower energy or concentration.
Diffusion can occur in any medium, whether it is liquid, solid, or gas.	Osmosis only occurs in a liquid medium.
Diffusion does not require a semipermeable membrane.	Osmosis requires a semipermeable membrane.
Concentration of the diffusion substance equalizes to fill the available space.	Concentration of solvent does not become equal on both sides of the membrane.
Hydrostatic pressure and turgor pressure to not normally apply to diffusion.	Hydrostatic pressure and turgor pressure oppose osmosis.
Does not depend on solute potential, pressure potential, or water potential.	Depends on solute potential.
Diffusion mainly depends on the presence of other particles.	Osmosis mainly depends on the number of solute particles dissolved in the solvent.
Diffusion is a passive process.	Osmosis is also a passive process.
The movement in diffusion is to equalize concentration (energy) throughout the system.	The movement in osmosis seeks to equalize solvent concentration (although it does not achieve this).

How does Reverse Osmosis work?

Reverse osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi--‐permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.

Seawater with TDS of 35,000 mg/L is considered standard seawater constituting, by far, the largest amount of water worldwide.  however, vary within wide limits from the Baltic Sea with 7,000 mg/L to the Red Sea and Arabian Gulf with up to 45,000 mg/L.

Solids are found in streams in two forms, suspended and dissolved. Suspended solids include silt, stirred-up bottom sediment, decaying plant matter, or sewage-treatment effluent. Suspended solids will not pass through a filter, whereas dissolved solids will.

Terminology

1.    Turbidity

Foreign suspended particles in water imparting an unsightly appearance and will result in deposits in water lines, process equipment, etc. It is measured by a Nephelometer that which senses the quantity of light transmitted through a water sample. The units are given as NTU.

2.    TOTAL DISSOLVED SOLIDS  (TDS)

 are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm).

3.    Conductivity

It is the reciprocal of electrical resistivity (ohms). Therefore conductivity is used to measure the concentration of dissolved solids which have been ionized in a polar solution such as water. The unit of measurement commonly used is one millionth of a Siemen per centimeter (micro-Siemens per centimeter or µS/cm).

TDS = Electrical Conductance X f

Where f = factor varies from 0.55 to 0.70  for different type of water sample

4. SILT DENSITY INDEX

The SDI is a popular method for determining the feed water quality in a RO

                                 SDI = 100 * (1 – Ti / Tf) / Tt

5.    Free Chlorine

Free chlorine refers to both hypochlorous acid (HOCl) and the hypochlorite (OCl–) ion or bleach, and is commonly added to water systems for disinfection.  Typical levels of free chlorine in drinking water are 0.2 - 2.0 mg/L Cl2, although regulatory limits allow levels as high as 4.0 mg/L.

6.  Recovery

 It is the ratio permeate flow to the feed water flow     

              Recovery = Permeate flow/Feed water flow

7.    Scale

 is a coating that forms on surface of membranes due to the precipitation or crystallization of salt compound or solids. Precipitate that forms on surfaces in contact with water as the results of a physical or chemical change, often due to the presence of calcium carbonate (CaCO3) or magnesium carbonate (MgCO3).

8.  Anti-Scalant

 is a chemical agent added to the RO feed water to inhibit the precipitation or crystallization of salt compounds