An overview of the Water purification process, Ozone treatment and Biological Activated Carbon
What is water purification?
Water purification is the process through which organic and inorganic pollutants from contaminated water sources are removed so that the water is made fit for a specific purpose.
Generally water purification plants treat water to remove unwanted components for the potable uses of water, home use or water for other purposes such as medical, pharmacological, chemical or industrial uses.
The steps taken in the water purification process
These are the steps usually taken to purify water from sources such as groundwater, lakes and reservoirs, rivers, etc.
Pre-treatment processes include pumping the water from source into pipes or holding tanks, removal of large debris such as plants, wood or fish through screening so that they do not interfere with later purification processes, storage of water to allow natural biological purification processes to occur, pre-chlorination to minimize the growth of contaminating organisms. The pre-chlorinating process has been discontinued, largely due to the potential adverse effects on the water quality.
They may utilize the following processes after the pre-treatment phase:
Adjusting the pH levels of the water
The pH level of acidic water from source can be raised by the addition of lime, soda ash or sodium hydroxide or sometimes the pH level of alkaline water can be reduced by addition of acidic compounds such as carbonic acid, hydrochloric acid or sulfuric acid.
Alkalinity of the water helps make coagulation and flocculation processes work effectively and reduces the risk of lead (from the pipes) dissolving into the water. But the drawbacks include:
- Addition of lime raises the hardness of water due to increased calcium ion concentration.
- Alkaline water may dissolve lead and copper from the plumbing system.
Precipitation of calcium carbonate or the dissolving of toxic metals is determined by the function of pH, mineral content, temperature, alkalinity and calcium concentration.
Coagulation and flocculation
Coagulation: The addition of coagulants is to assist in the removal of both inorganic and organic impurities such as clay, silt, algae, bacteria and other organic particles which contribute to turbidity and color of water.
Flocculation: This occurs when inorganic coagulants cause simultaneous chemical and physical interaction among the particles. The inorganic coagulants neutralize the negative charges on the particles. Through this process, floc, which is “a loosely clumped mass of fine particles”, is formed.
Both the coagulation and flocculation processes occur right after the addition of coagulants.
Sedimentation: Now the water may enter the sedimentation basin where the floc is allowed to settle to the bottom.
Sludge is generated from the sedimentation process which should be cleared.
Floc blanket clarifiers are used to remove particulates by entrapping them in a layer of suspended floc while forcing the water upward.
Dissolved air flotation
The additional process of dissolved air flotation (DAF) generates a concentrated, floating mass of floc by creating fine bubbles that attach to the floc. The bubbles are created by the air diffusers at the bottom of the bottom of the DAF tanks.
Rapid sand filter diagram
Filtration: This step is to finalize the further removal of suspended particles and floc that is still unsettled. Rapid sand filters use granular media to allow removal of particles and impurities that are trapped in the floc. Based on the functioning method, there are two types of rapid sand filters viz. the gravity type and the pressure type.
The gravity type uses gravity to aid flow of unfiltered water into filter medium and the pressure type uses pumped pressure. The floc gets trapped in the sand matrix.
Slow sand filters require more land and space than rapid sand filters. They use biological treatment processes. A thin biological layer called Schmutzdecke, a hypogeal layer is developed on the surface of the slow sand filter. The Schmutzdecke, typically consists of a gelatinous biofilm with bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. As this biofilm ages, more algae develop and larger aquatic organisms such as bryozoa, snails and Annelid worms are present. This biofilm traps the foreign matter from the flowing water and allows adsorption of soluble organic material. Contaminants are metabolized by the microorganisms.
Membrane filters may be used for tertiary treatment of water. These filters remove particles larger than 0.2 μm.
In this process, pathogens such as viruses, bacteria or protozoa that pass through the filters are killed by the addition of disinfectant chemicals such as chlorine, chloramines or ozone.
There are also other disinfection methods such as ultraviolet disinfection or solar disinfection, etc.
Click this link for a short PDF about the Asaka water purification plant
General insight into the process at the Asaka water purification plant in Tokyo
Here is an insight into water purification plant process at the Asaka water purification plant in Tokyo.
- Water from river is taken in before it is passed through sand basins.
- In the sand basins, sand and soil from the river water settle down.
- Intake pumps pump the water into a receiving well.
- The river water is stabilized and its level and flow are adjusted before the water is lead into sedimentation basins.
- Chemical agents facilitate the sedimentation of the water by turning the pollutants into floc.
- Coagulated floc is then allowed to sediment.
- Rapid sand filters are used for the sand filtration process of the flowing water.
- For advanced water treatment, the water is pumped into ozone contact basins where the water undergoes ozonation. In the ozone contact basins, the ozone oxidizes the water so that organic substances are decomposed and complications such as musty odors are removed.
- In the next process, the water is passed through biological activated carbon adsorbing basins. In biological activated carbon (BAC), the activated carbon part adsorbs the pollutants and the biological part consisting of the microorganisms decompose the contaminating organic substances, also removing odor and bad taste.
- The water undergoes sand filtration again.
- Chlorine is injected to further disinfect the water.
- The purified water is then stored in reservoirs before it can be pumped to water supplying stations.
- The operating room always monitors each and every step of water treatment and the mechanical conditions of the purification plant. The water levels at each step are also adjusted.
Ozone treatment or ozonation of water
- Ozone for water treatment is produced by passing oxygen through ultraviolet light (UV-type ozone generators or through an electric discharge field (corona discharge-type ozone generators). Ozone generation is associated with hazards such as fire hazards or toxicity and so careful consideration is required.
- Ozone is effective over an extensive range of pH levels of the water to be treated. As an oxidizing agent, it reacts with organic matter and inorganic matter, microorganisms such as bacteria, protozoa and viruses.
- This process generates biodegradable organic matter.
- Ozone, when compared to chlorine, leaves behind lesser amounts of unsafe by-products (no residual chemical remains), reduces the problem in taste and odour but that does not ensure that using ozone is completely safe or that ozone is better than chlorine. Ozonation can produce by-products which might be carcinogenic even though they may be less in amount. Production of by-products is the reason for the inclusion of activated carbon filters.
This is how activated carbon looks like
Biological Activated Carbon process
- This process combines ozonation and the usage of granular activated carbon (GAC).
- GAC is a form of activated carbon having a high surface area and able to adsorb toxic compounds, gases and vapors.
- When used for water treatment, GAC is able to adsorb organic matter (including biodegradable organic matter), taste and odor causing compounds.
Do you consider tap water safe for drinking?
An example for the water treatment process
© 2015 Arun Dev