What's the cheapest way to get a glass of clean water? The answer may surprise you. Especially if, like many people, you are under the impression that powdered activated carbon (PAC) is more cost-effective than granular activated carbon (GAC) in a typical water treatment process .
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Over the last decade a number of articles have been published discussing the cost of installed GAC treatment systems. The conclusion reached by the majority of these studies is that GAC is cost prohibitive and that PAC represents a clear savings. A study by Adams and Clark, for instance, indicates that the total cost estimate for GAC systems ranges from 10 cents to $1.00 per 1,000 gallons of water, depending on the size of the system (specifically 150 mgd to 0.1 mgd respectively).
Those numbers probably are enough to make the managers of most water treatment plant think twice about implementing GAC, no matter if the GAC-treated water may taste and smell better. But a recent survey of drinking water facilities across the U.S. which use GAC revealed some interesting economic results. The study was conducted by Calgon Carbon Corporation, a company which makes and markets both PAC and GAC.
Not So Fast
What isn't immediately apparent is that research studies like this have been based on GAC treatment facilities which had been constructed from the ground up. They lump the cost of GAC with high construction expenses-excavation and site work, manufactured equipment, concrete, steel, labor, pipes and valves, electrical components, and instrumentation. Even the U.S. EPA, in a recent study, estimated capital and operations and maintenance (O&M) costs for complete treatment facilities without considering existing filtration systems (green-field installations).
In reality, however, today very few water treatment plants are built from the ground up with the activated carbon process in the design. The fact is, most U.S. plants using activated carbon have retrofitted existing facilities. Retrofitting puts cost comparisons between GAC and PAC in a more realistic light.
Real Numbers from Real Plants
Water facilities across the country which have been retrofitted to incorporate GAC were recently surveyed regarding the actual costs they have experienced. The results show most facilities incurred little, if any, capital costs to retrofit their plants. The majority of sites required only a replacement of the current media (typically sand/anthracite) with GAC. While the height of the filter troughs usually had to be increased to accommodate the proper quantity of carbon and achieve the necessary contact time, the cost for this was reported to be insignificant.
Comparing Apples with Apples
Since retrofitting eliminates or drastically reduces the capital cost associated with GAC, how does the technology stack up against PAC in terms of day-to-day operating expenses?
Consider that the typical PAC water treatment facility, using an average of 3, 5 or 7 mg/l of powdered carbon annually, spends between $31,000 and $75,000 per year to produce 10 mgd of treated water. Since GAC continues to provide taste and odor control for three years, it is necessary to first annualize the cost for GAC treatment, then compare those numbers with the PAC numbers.
The cost comparison results are shown in the figure. At a typical empty bed contact time (EBCT) of 5 minutes, the cost of GAC is actually comparable to PAC at standard dosages. Obviously, the numbers vary based on the EBCT and PAC dosage level, but-for the majority of the water plants responding to the survey-the additional cost calculated per customer family lies between 1/4 to 1/2 cent per day for either PAC or GAC.
If your process currently includes powdered carbon treatment, a quick rule of thumb can be applied to estimate the GAC conversion quantities. Granular carbon is roughly three times more effective on a weight-basis than PAC for taste and odor control. That means if you currently use 90,000 pounds of PAC, you can achieve the same quality (or better) of water with 30,000 pounds of GAC.
Go with the Flow
Many water plant operators assume that, even with GAC in place, they will still need some level of PAC dosage. This is not the case. At typical dosages, a granular carbon bed can completely replace powdered carbon, producing a comparable, if not superior, quality of water during the life of the carbon.
To determine an accurate PAC vs. GAC cost comparison for a particular facility, you must also consider the influent conditions of the water and the ultimate treatment objective. A report prepared by Environmental Engineering & Technology (EE&T), a consulting firm, for Erie County Water Authority in Pennsylvania concludes that the use of GAC filter media is cost-effective. The engineers based their findings on comparing GAC filter usage with PAC dosages ranging from 15 mg/l to 30 mg/l, for a 70 mgd plant flow at 2.0 and 1.0 year replacement frequency.
EE&T's EBCT values in these studies averaged two minutes. In normal circumstances, at least a five minute EBCT is recommended for effective treatment over several years. It is better to operate at the five-minute contact time and have the carbon on-line three years, instead of for only one year at a two-minute EBCT.
Other Differences
As a result of installing GAC, retrofitted plants surveyed reported a minor savings in sludge disposal costs and utility costs (for running the mixer for the PAC slurry).
Their most significant issue in terms of savings, however, centered on chemical cost changes. After installing GAC, the use of chemicals-including disinfectants (primarily chlorine), coagulants/polymers (aluminum sulfate, sodium sulfate, ferric sulfate) for turbidity removal, and potassium permanganate for taste and odor control-declined substantially.
For example, the City of Cincinnati reduced chlorine costs by two thirds when a switch was made to GAC. Staff estimated an incremental cost per family of $22 annually, or six cents per day. Another city reported that the decrease in chemical costs was so significant that the move to GAC actually resulted in a net savings.
Beyond Budget
So, if the cost of GAC and PAC are comparable, why choose granular carbon over powdered?
The Bottom Line
In the real world, water utilities do not generally build new GAC facilities, they retrofit existing sites-at minimum expense. Data shows that the incremental cost of installing GAC is usually less than 1/2 cent per day per customer family, with the high end at 6 cents per day per family. (The high end cost, as in the Cincinnati case, factors in the construction costs for an on-site carbon reactivation facility.) After the installation of a GAC system, not only is PAC dosage eliminated or significantly reduced, the consumption of other treatment chemicals also decreases measurably. And the water is said to taste and smell better too.
What's the cheapest way to get a glass of clean water? When the actual numbers are annualized and compared, and related issues of waste and maintenance, and individual site specifics and objectives, are all factored in, granular activated carbon could very well be the cost-effective alternative.
About the Authors: This article was written by Annette Vickers, Marketing Manager-Municipal and Gary Van Stone, Business Director-Municipal, both with Calgon Carbon Corporation in Pittsburgh, Pennsylvania; . 412-787-; Fax. 412-787-.
Turbidity is a cloudy appearance of water caused by small particles suspended therein. Water with little or no turbidity will be clear.
Turbidity is not only an aesthetic problem in water. Water with a high turbidity can be very difficult or impossible to properly disinfect. As a result, the maximum allowable level of turbidity in water is 0.5 NTU, while the recommended level is about 0.1 NTU. (NTU, or TU, stands for nephelometric turbidity units, a measurement of the turbidity of water.)
In addition to removing turbidity from the water, coagulation and flocculation is beneficial in other ways. The process removes many bacteria which are suspended in the water and can be used to remove color from the water.
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Turbidity and color are much more common in surface water than in groundwater. As surface water flows over the ground to streams, through streams, and then through rivers, the water picks up a large quantity of particles. As a result, while aeration is more commonly required for groundwater, treatment involving coagulation and flocculation is typical of surface water.
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. You can also learn more about our equipment partner Matec for use in dewatering after pretreatment.
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.
Coagulation is a somewhat simple chemical process that involves bringing insoluble materials together by manipulating the charges of particles, by adding iron or aluminum salts, such as aluminum sulfate or ferric sulfate, to a wastewater stream. The primary purpose of using a coagulant besides removing vary fine particles from suspension is that this process results also in less turbidity of the water, i.e. clearer water.
With coagulants’ positive charge, the negatively charged particles in the water are neutralized. This causes the suspended solids in the water to bind together into larger flocs. These larger flocs begin to settle at the base of the water supply. The larger the size of the particles, the quicker the floc settles.
Coagulation helps to remove a number of different pollutants that cause your water to become dirty or toxic, including:
Through coagulation, industrial water supplies are put into the perfect chemical state for easy mechanical filtration. Once the flocs settle at the bottom of your clarifier, equipment like a filter press can then take those larger clumps of aggregated particles and remove them, delivering clean water back into your system.
When used together, coagulants, clarifiers and filter presses offer maximum water reclamation of over 95 percent. With so little water actually discharged with the solids, you can create a nearly closed-loop process.
Organic coagulants are best used for solid-liquid separation. They are also good options to use when trying to reduce sludge generation. Being organic in nature, these coagulants offer the added benefits of working at lower doses and having no effect on the pH of your water.
Organic coagulants are typically based on the following formulations:
PolyAMINEs and PolyDADMACs – These cationic coagulants work by charge neutralization alone and are the most widely used organic coagulants. PolyAMINEs and PolyDADMACs neutralize the negative charge of colloids in your water, forming a spongy mass called a “microfloc.” Since they only coagulate through charge neutralization, they don’t offer any advantages in regard to the sweep-floc mechanism (explained later with inorganic coagulants).
Melamine Formaldehydes and Tannins – These natural coagulants work somewhat similarly to inorganic coagulants in that they both coagulate colloidal material in the water and also contribute their own precipitated floc. This sweep-floc precipitate can absorb organic materials such as oil and grease while coagulating unwanted particles together in your water. Since the precipitate dewaters everything to low moisture concentration, these coagulants are great for operations that generate hazardous sludge, such as what’s found in oil refineries.
The main advantages of organic coagulants include; lower dosage, lower volume of sludge produced and no effect on the pH.
Inorganic coagulants are typically cheaper than their organic counterparts, making them a cost-effective solution for a broad range of water treatment applications. They are especially effective when used on raw water with low turbidity.
When added to water, inorganic coagulants form aluminum or iron precipitates. These help to clean the water by absorbing impurities in the water as they fall. This process is known as the “sweep-floc” mechanism. However, this can add to the overall sludge volume that must be treated and removed, so it’s not the right choice in every scenario.
The main types of inorganic coagulants include:
Aluminum Sulfate (Alum) – As one of the most common water treatment chemicals used in industrial processes, alum is the go-to coagulant choice for many. Manufactured as a liquid, alum’s crystalline form is created when the liquid is dehydrated. It should be noted that alum is mildly hazardous and has similar health effects/corrosion characteristics as diluted sulfuric acid.
Aluminum Chloride – This coagulant works similarly to alum, but it’s more expensive, hazardous and corrosive. As such, it’s usually only picked as a second choice in processes where alum could not be used.
Polyaluminum Chloride (PAC) and Aluminum Chlorohydrate (ACH) – These inorganic coagulants are best used for more basic water supplies.
Ferric Sulfate and Ferrous Sulfate – While ferric sulfate is more commonly used, both iron coagulants work similarly to aluminum coagulants. Ferrous sulfate is typically a good choice in applications where you need a reducing agent or excess soluble iron ions are required.
Ferric Chloride – Since it is generated as a waste material from steel making operations, ferric chloride is the least expensive inorganic coagulant. However, it’s only used in facilities that can handle its reputation as the most corrosive and hazardous inorganic coagulant.
Once you have the right coagulant, you add these chemicals to your dirty water and mix rapidly. That way, the coagulant is quickly and easily circulated throughout the water.
The residuals or by-products of these coagulants generally do not pose a water quality issue, so long as they were applied properly and with the right dosage. This is why having a water treatment expert is key. Professionals who are experienced with wastewater treatment can even set up the coagulation process so that the coagulant chemicals are removed with the floc during filtration.
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