Hard water is water that has high mineral content. The name comes from the hard scale that forms when this water is boiled. It is not harmful to human health, but in the industrial setting it causes costly breakdowns in boilers, cooling towers, and other water handling equipment. The divalent cations Mg and Ca are the most frequently found minerals causing hard water. These ions enter water supplies by leaching from minerals in underground aquifers.
Cold lime softening is the addition of chemicals to remove the calcium and magnesium ions by precipitation. Cold (ambient temperature) lime softening is used when the mineral content of the source water ranges from 150 ppm to 500 ppm. Treatment consists of five steps: Chemical Pretreatment, Clarification, Recarbonation, Filtration, and Sludge Treatment.
Chemical Pretreatment
Sodium hypochlorite (bleach) is added to kill any living organisms that may be in the raw water.
Coagulant (usually ferric or aluminum salts) is sometimes added to aid particles to come together. Polymer acts as glue and turns individual particles into larger clusters. The larger particles settle faster and form a more concentrated sludge.
Softening and Clarification
Softening is the removal of calcium and magnesium from the water that could leave hard water deposits called “scale.” After the chemical pretreatment, the water flows into the Solids CONTACT CLARIFIER™ softener. The softener provides a reaction zone for the hardness causing ions to precipitate. The lime and (sometimes) soda ash are added in the reaction zone just before the feed enters the clarifier. Lime is added to increase the pH to remove calcium and magnesium hardness.
Soda ash may be added to remove calcium hardness if the raw water doesn’t contain sufficient alkalinity for complete removal of hardness. Settled solids react with lime and soda ash to form larger and faster settling particles. The clarified water flows over weirs and the solids (sludge) are scraped to the center for removal and dewatering. The overflow typically has less than 10 mg/L of suspended solids. The amount of hardness remaining will depend on the water chemistry and proper chemical addition.
Recarbonation
After softening, the pH is lowered to less than 8 to stop the softening reaction. This prevents what is called “post-precipitation” where softening continues in the pipe and eventually the pipe fills with scale. Clarifier overflow may be treated by filtration. As the water flows to the WWETCO FlexFilter™, polymer may be added to improve filtration. Filtered water typically has less than 1 mg/L of suspended solids. Intended use directs the stream either to a cooling tower or further filtration.
Sludge Treatment
Solids CONTACT CLARIFIER underflow is pumped into a thickener. Typically underflow solids are much denser than the clarifier feed with 3-4 percent solids being typical. Polymer may be added to improve settling and clarity. The thickened underflow is sent to a belt press or other type of vacuum or pressure dewatering filter. The belt press places the solids between two belts and squeezes the water out through a series of rolls. The belt press requires additional polymer to function properly. The resulting cake is then hauled off for disposal. The pressate and cloth wash water are pumped to the wastewater recovery sump along with the ultrafiltration system’s backwash and the reverse osmosis reject before recycling into the clarifier.
Surface Water Pretreatment
Surface water pretreatment prepares water for use in any type of treatment plant (i.e., power, chemical, petrochemical, etc.) and is needed when the source of water comes from a raw/contaminated source (usually river water) where the total suspended solids (TSS) can range from 50 mg/L to 200 mg/L. This treatment consists of four steps: chemical pretreatment, clarification, filtration, and sludge treatment.
Chemical Pretreatment
Sodium hypochlorite (bleach) is added to kill any living organisms that may be in the raw water. Coagulant helps particles come together to improve clarity and settling. Polymer turns individual particles into larger clusters. The larger particles settle faster and form a more concentrated sludge.
Clarification
RapiSand™ Ballasted Flocculation is a high-rate water clarifying system utilizing both chemical and physical treatments to remove suspended solids and unwanted particles. The process uses the proven flocculation technique of adding a dense ballast sand, allowing for much higher settling rates. Raw water is mixed with a coagulation agent to destabilize and neutralize particles in the water. During the next step of flocculation, sand and polymer are combined with the coagulated flow. The flocculation mixer provides the particles with enough energy to stay suspended and grow in size.
In the sedimentation area, developed floc particles (coagulant, polymer, sand, and solids) are allowed to settle to the bottom. Clarified water may pass through tube settlers or inclined plate settlers before it is drawn off the top of the sedimentation basin. The settled solids are gathered and pumped from the sedimentation area through a hydrocyclone where the sand is separated for reuse and the solids are processed further.
Filtration
If necessary, the overflow can be treated by filtration. The process starts in an equalization tank that allows for a constant flow into the dual media filters. As the water flows from the RapiSand to the dual media filter, polymer may be added to improve filtration. The resulting water is now less than 1 mg/L TSS. It may be directly sent to a cooling tower or it can be further filtered depending on the intended use.
If the water is being used for a boiler then it may need to go through reverse osmosis. An ultrafiltration membrane system removes small suspended solids from the water to improve the efficiency of the reverse osmosis system.
Sludge Treatment
The underflow of the RapiSand is pumped into a thickener. Polymer is added to improve settling and clarity. The thickened underflow is sent to a filter press or other type of vacuum or pressure dewatering filter. The slurry is pumped into the filter press under pressure to force the moisture out. The filter press may require additional polymer to function properly. The cake is then hauled off for disposal and the pressate and wash water is recirculated for further use. The pressate and cloth wash water get pumped to the reclaim water storage sump with the backwash from the filtration systems.
