How Industrial Wastewater Pretreatment Works
Many industrial plants and facilities use or create wastewater with their processes. The wastewater can be discharged, based on local regulations and permit requirements, or re-used within their process. Most likely in all cases, the water needs to be treated to some degree to meet quality characteristics or regulatory requirements – often referred to as wastewater pretreatment.
The term “pretreatment” means the treatment of wastewater by commercial and industrial facilities to remove harmful materials before being discharged to a sewer system under the control of a publicly owned wastewater treatment plant. If you fail to properly treat your water or improperly manage your discharge, you could incur fines and possible legal action. Apart from avoiding regulatory penalties, wastewater pretreatment is likely essential to the quality of your products.
At ChemREADY, we understand the unique needs of wastewater pretreatment for industrial applications. Industrial wastewater pretreatment differs from treating municipal sewage in that you must focus on the specific process that creates the waste stream. The more wastewater you produce and discharge, the bigger the wastewater surcharge bill from the city. However, by pretreating your wastewater, you can help cut costs and preserve profits – so wastewater pretreatment can impact your business’ bottom line just like plant production can. Even if you plan to recycle the water for your plant’s processes, the following pretreatment methods are effective at maintaining your water’s quality and ensuring its reuse.
What Is Pretreatment Of Wastewater
The pretreatment process usually starts with an evaluation of the water and an assessment of the discharge circumstances. If we assume a generic wastewater and the specifications of the discharge permit is obtainable, then pretreatment likely will follow a basic course of treatment. The goal of pretreatment is to remove waste solids in the water, including:
- Suspended and dissolved solids
- Synthetic chemicals
Pretreatment normally involves the use of chemicals and mechanical filtration (or dewatering) methods.
What Chemicals Are Used In Wastewater Treatment Plants?
When considering your pretreatment method, the first place to start is determining what you need to remove from the water to meet discharge requirements. Many industrial applications create microscopic materials small enough that they can get through mechanical filtration means. However, with appropriate chemical treatment, you can remove ions and smaller dissolved solids from your water, as well as the suspended solids. The top three types of chemical pretreatments for wastewater involve:
- pH Adjusters
First, you need to insolubilize any metals ions and chemicals you need to remove using pH. A coagulant is then used to bring the fine solids to a point that they form a pin-floc. A pin-floc can be described as a small, weak floc that is subject to settling in a clarification process. A flocculant is then added to facilitate rapid settling or floatation in some cases, and then feed the clarification process to allow for dewatering to occur.
To remove metals and other dissolved contaminants from wastewater, pH adjusters can be used. By raising the pH of the water, and thus the number of negative hydroxide ions, it will cause positively-charged metal ions to bond with these negatively-charged hydroxide ions. This results in a dense and insoluble metal particle that can be filtered out.
Learn more about our pH and Alkalinity adjusters in our Water Facts blog.
For any wastewater pretreatment program that deals with suspended solids, coagulants can consolidate suspended contaminants for easy removal. Chemical coagulants used in industrial wastewater pretreatment fall into one of two categories: organic and inorganic coagulants.
Inorganic coagulants are cost-effective and can be used in a wider variety of applications. They are especially effective on any raw water with low turbidity, an application not fit for organic coagulants. When added to water, inorganic coagulants from aluminum or iron precipitates that absorb the water’s impurities, thus cleaning it. This is referred to as the “sweep-floc” mechanism. While effective, this process adds to the overall sludge volume that needs to be removed from the water. Frequently used inorganic coagulants include aluminum sulfate, aluminum chloride, and ferric sulfate.
Organic coagulants offer the advantages of lower dosages, lower volumes of produced sludge, and no effect on the pH of the treated water. Examples of common organic coagulants include polyamines and polyDADMACs, as well as melamine formaldehydes and tannins.
Used in a wide range of industries and applications, flocculants help to remove suspended solids from wastewater by aggregating contaminants into flakes or “flocs” that float to the surface of the water or settle at the bottom. They can also be used for lime softening, sludge thickening, and solids dehydration. Natural or mineral flocculants include activated silica and polysaccharides, while synthetic flocculants are most commonly based on polyacrylamide.
Depending on the charge and chemical composition of your wastewater, flocculants can either be used on their own or in combination with coagulants. Flocculants differ from coagulants in that they are often polymers, whereas coagulants are typically salts. They can range in molecular size (weight) and charge density (% of the molecule with either anionic or cationic charges), which is used to “balance” the charge of the particles in the water and cause them to come together and dewater. Generally speaking, anionic flocculants are used to catch mineral particles while cationic flocculants can capture organic particles.
At ChemREADY, our high-quality FlocREADY line of flocculants and coagulants are designed to improve wastewater treatment and lower overall costs in a wide range of mineral processing applications. We offer a wide range of cationic, anionic, and non-ionic flocculants, we well as organic and inorganic coagulants for all of your chemical treatment needs.
After chemical pretreatments, filtration systems can remove suspended solids and larger contaminants from your water. Filtration can be done immediately following chemical treatment or following a settling clarification step that uses a silo or tank to allow for floating, settling and/or dewatering. While there are multiple ways to filter out contaminants, the three main types of mechanical filtration equipment are:
- Belts presses
- Filter presses
Each technology has associated CAPEX and OPEX characteristics that can be matched to application details, geographic location and project constraints. The following will outline some of these constraints.
Used since the 1930s to treat industrial wastewater, centrifuges use the force from a rapidly rotating cylindrical bowl to remove solids from a liquid. This high-speed process effectively cleans your water through the centrifugal thickening and dewatering of sewage sludge. By removing water, the thickening process can help reduce the amount of tankage needed for dewatering and storage. Dewatering then separates even more water, the centrate, leaving you with a thickened material known as thickened solids, or dewatered solids (dws), which can be separately disposed of. Centrifuges require almost twice the amount of energy and flocculant to operate than other means of dewatering. The resulting costs can be prohibitive to implement (see Table 1).
Belt filter presses, more simply referred to as belt presses, are continuous solids dewatering devices used for waste removal. In use since the 1970s, belt presses primarily dewater sludge through a combination of gravity drainage and mechanically applied pressure. In a typical belt press, wastewater is first introduced into the gravity drainage section, where the slurry thickens as free water is removed. The slurry’s concentration of suspended solids often increases by a factor of three here. Then, the press will apply a gradually increasing pressure to the slurry by squeezing it between opposing belts held in place by a series of rollers. These rollers apply shearing forces to the sludge as the belts move through several off-set rollers, each with a smaller diameter than the last.
The combination of these squeezing and shearing forces removes even more water until you have a concentrated amount of dewatered solids. Belt Presses almost always require additional flocculant and coagulant be added to facilitate the dewatering process. Also, they tend to require lots of labor hours to operate and maintain. These lead to additional operational costs (Table 1). They do however provide a continuous process for dewatering instead of a batch-like process.
Recessed chamber filter presses use pump-induced pressure to filter out solids from wastewater. In a filter press, sludge is pumped at high pressure through a plate’s filter clothes until contaminants are separated from the rest of the water.
The plate and frame, or recessed-plate filter press is one of the oldest proven types of dewatering equipment, producing cakes with the highest concentration of solids for any mechanical dewatering device. Their filters are typically made of polypropylene squares with concave depression and a hole in the middle. Two of these plates are joined to create a pressurized chamber that forces out liquid through a filter cloth lining the chamber without the need for additional chemical, leaving you with a filter “cake”. The filter press can be fully automated in its modern form and when implemented be one of the most reliable methods available for dealing with solids in water (Table 1).
Table 1: Filtration Operating Costs & Resources
|For every 20 Tons / Hour||Filter Press||Belt Press||Centrifuge|
|Flocc./Coag.||$ –||$ 30,281||$ 77,078|
|Energy||40 Kw/h||40 Kw/h||75 Kw/h|
|Spare Parts||$ 15,000||$ 25,922||$ 28,675|
|Disposal Handling||1 Stage||2 Stage||2 Stage|
|Operator/Maint.||200 Hr.||2000 Hr.||500 Hr.|
|Maint. Cost @ $25/Hr.||$ 5,000||$ 50,000||$ 12,500|
|Total Projections:||$ 20,000||$ 136,203||$ 148,253|
For filtration pretreatment, one-size-fits-all does not really exist. The material being filtered plays a large roll in method of choice. In some cases, any one of the methods may feasibly work better than others. For example, in aggregates and mining, the Filter Press is the likely favorite. In municipal wastewater, belt presses are often employed because the material does not filter well on a filter press. More recently, we are seeing centrifuges used in a variety of non-traditional settings like recycling plants. At ChemREADY, we focus primarily on Filter Presses due to the reduced operating costs, but we are able recommend pretreatment options for all three.
The ChemREADY Advantage
ChemREADY is your go-to partner for wastewater treatment management. Along with offering the best chemicals and products for your wastewater pretreatment process, our team has the expertise to make sure that you’re getting the perfect solution for your unique needs.
If you want to learn more about how we can help with the wastewater treatment process, or you’d like to discuss your needs in detail, contact the ChemREADY team today.