|Figure 1: Conventional treatment.|
Conventional treatment consists of the following unit processes: coagulation, flocculation, clarification, and filtration, and is typically followed by disinfection at full-scale. Figure 1 describes conventional treatment. Conventional treatment is often preceded by pre-sedimentation, may be accompanied by powdered activated carbon (PAC) addition, utilize granular activated carbon (GAC) as a filter media, and in some cases be followed by GAC adsorption. Conventional treatment is often preceded by pre-oxidation, or oxidation takes place concurrently. Oxidants common to conventional treatment are chlorine, chloramine, chlorine dioxide or permanganate. Occasionally membrane processes, either membrane filtration or ultrafiltration, accompany conventional treatment.
In coagulation, a positively charged coagulant (usually an aluminum or iron salt) is added to raw water and mixed in the rapid mix chamber. The coagulant alters or destabilizes negatively charged particulate, dissolved, and colloidal contaminants. Coagulant aid polymers and/or acid may also be added to enhance the coagulation process. Turbidity and total organic carbon (TOC) are measures of particulates and dissolved organics impacting coagulation.
During flocculation, gentle mixing accelerates the rate of particle collision, and the destabilized particles are further aggregated and enmeshed into larger precipitates. Flocculation is affected by several parameters, including the mixing speed, mixing intensity (G), and mixing time. The product of the mixing intensity and mixing time (Gt) is frequently used to describe the flocculation process.
There are two primary destabilization mechanisms in drinking water treatment: charge neutralization and sweep flocculation. The mechanism is dependent upon the coagulant dose. Most drinking water treatment plants operate using sweep flocculation, which requires a higher coagulant dose, rather than charge neutralization. In charge neutralization, the positively charged metal coagulant is attracted to the negatively charged colloids via electrostatic interaction. Flocs start to form during the neutralization step as particle collisions occur. Adding excess coagulant beyond charge-neutralization results in the formation of metal coagulant precipitates. These metal hydroxide compounds (e.g., Al(OH)3 or Fe(OH)3) are heavy, sticky and larger in particle size. Sweep flocculation occurs when colloidal contaminants are entrained or swept down by the precipitates as they settle in the suspension.
The optimal pH range for coagulation is 6 to 7 when using alum and 5.5 to 6.5 when using iron. For high alkalinity water, excessive amounts of coagulant may be needed to lower the pH to the optimal pH range. In these cases, it may be beneficial to use acid in addition to the coagulant to reduce the amount of coagulant needed and effectively lower chemical costs. Enhanced coagulation is now widely practiced for removing disinfection byproduct (DBP) precursors, and it also removes inorganics, particulates, and color causing compounds. Removing these contaminants using coagulation depends on the amount of coagulant added. It is important to determine the optimal dose for coagulation; insufficient doses will not effectively destabilize the particles and adding excessive doses can cause detrimental effects such as re-stabilization, excessive sludge production, or corrosion.
Water quality parameters such as pH, temperature, and alkalinity may dictate effectiveness of the coagulation-filtration process. The pH during coagulation has a profound influence on the effectiveness during the destabilization process. The pH controls both the speciation of the coagulant as well as its solubility, and it also affects the speciation of the contaminants. For high alkalinity water, an excessive amount of coagulant may be required to lower the pH to the optimal pH ranges (alum pH 6 to 7, iron 5.5 to 6.5). Temperature also impacts the coagulation process because it affects the viscosity of the water. Thus lower temperature waters can decrease the hydrolysis and precipitation kinetics. For some treatment objectives, other parameters like iron, manganese or sulfate impact coagulation. Some of the alternative coagulants such as polyaluminum chloride (PACl) can be advantageous over the traditional coagulants in low temperature conditions as these coagulants are already hydrolyzed, and therefore temperature tends to have less effect on the coagulation process.
Following flocculation, agglomerated particles enter the clarification unit where they are removed by sedimentation by gravity or are floated and skimmed from the surface of the clarification unit. In the sedimentation processes, the majority of the solids are removed by gravitational settling; particles that do not settle and are still suspended are removed during the filtration process. Sedimentation is generally accomplished in rectangular or circular basins and is often enhanced by the addition of inclined plates or tubes which increase effectiveness of the process by effectively increasing the surface area of the sedimentation basin. Dissolved air flotation (DAF) is another clarification process in which air is diffused as fine bubbles and suspended particles are floated to the surface and removed by skimming. Generally, DAF is most effective for small, fine, low-density particles like algae and may not be effective is all instances. Like conventional sedimentation, solids not removed by DAF are removed during filtration.
Two parameters frequently used to describe the clarification process are the overflow rate and the detention time. The overflow rate is the process loading rate and is usually expressed in gpm/sf or gpd/sf. Overflow rates for conventional sedimentation generally range from 0.3 to 1 gpm/sf (500 to 1500 gpd/sf). Overflow rates for other processes can vary significantly. There are proprietary sand-ballasted clarification systems that have been demonstrated to operate effectively at overflow rates as high as 20 gpm/sf. Typical detention times range from 1 to 2 hours, although many states require up to 4 hours for full-scale surface water treatment.
The most commonly used filter type in the conventional treatment process is a dual-media filter comprised of anthracite and sand; however, mono-media (sand), multi-media (garnet, anthracite, and sand), and other media configurations, including the use of granular activated carbon, are also used in drinking water treatment. During filtration, the majority of suspended particles are removed in the top portion of the filter media. Filters are backwashed to dislodge and remove particles trapped within the filter bed, to reduce head loss (pressure build up), and to keep the filter media clean.
The filter loading rate is a measure of the filter production per unit area and is typically expressed in gpm/sf. Typical filter loading rates range from 2 to 4 gpm/sf; however, higher filter loading rates, 4 to 6 gpm/sf, are becoming more common at full-scale. This can be a critical parameter because it determines the water velocity through the filter bed and can impact the depth to which particles pass through the media. The filter run time describes the length of time between filter backwashes during which a filter is in production mode. The filter run time is not only an indicator of the effectiveness of prior treatment (i.e., the ability of the coagulation and clarification steps to remove suspended solids), but also plays a role in the effectiveness of the filter itself. Filter performance, particularly with regard to particulate contaminants, is often poorest immediately following a backwash. As the filter run time increases and the concentration of solids in the media increases, the filtration process often performs better with regard to particulate contaminant removal.
Residuals generated by the conventional treatment process include coagulation solids (sludge) and spent backwash. Spent backwash is often returned to the treatment process as a means to minimize water loss. Sludge may also be recycled to minimize coagulant and coagulant aid doses and improve process performance. Process solids (i.e., coagulation sludge and filtered solids) will contain elevated concentrations of contaminants removed during the treatment process. Depending on the source water concentration of a particular contaminant and any disposal limitations, it may be necessary evaluate the disposal of process solids with respect to state and local hazardous waste regulations.