As of mid-August 2012, drought conditions plagued over 60% of the contiguous 48 states. Almost one-quarter of the country experienced extreme to exceptional drought. This has brought the need to reduce freshwater withdrawals into sharp focus for many municipalities and those who depend on private wells for their water supply.
The demand for potable water has been steadily on the rise due to the growth of a thirsty urban population. The number of people living in urban areas is projected to increase from 2.85 billion in 2000 to 4 billion in 2020; a 40% increase. As a result, the municipal treatment plants are struggling to keep up with the demand.
Increasing demand coupled with a limited supply of fresh water are driving the growing practice of wastewater reuse. Currently, 1.7 billion gallons of wastewater are reused every day. And this practice is growing by 15% each year. Florida, California, Texas and Arizona are the biggest reusers of treated wastewater, but other states are joining in as the demand for fresh water outstrips the available supply.
The persistent notion that all water should be treated to achieve potable water quality is fading. Many applications do not require higher level water quality that meets drinking water standards. This is particularly true in industrial settings where large volumes of water are required for utility and process applications. Table 1 (below) indicates the reclaimed water quality in Florida and Texas. This is typical of tertiary municipal wastewater in other states, as well. Although not of potable water grade, it is still a viable resource for other industrial requirements such as cooling water makeup, boiler feedwater and many process applications.
New and improved water treatment processes are emerging to meet the increased need to recycle and reuse industrial wastewater. Membrane separation technology plays a key role in treating wastewater for recycle or reuse. This includes:
- Membrane bioreactors (MBR)
- Microfiltration (MF)
- Ultrafiltration (UF)
- Nanofiltration (NF)
- Reverse Osmosis (RO)
WASTEWATER TREATMENT WITH MEMBRANE BIOREACTORS
Membrane bioreactors are a modification of the classic activated sludge concept used for decades to treat municipal and industrial wastewater. Here a membrane process like microfiltration or ultrafiltration is used in combination with a suspended growth bioreactor. The membrane unit replaces the settler and sand filter used in a conventional activated sludge system.
Initial designs placed the membrane separator external to the bioreactor in a sidestream configuration. The effluent from the reactor is pumped through the membrane filter to separate the solids from the water. This design produces acceptable effluent quality, but poses operating problems with regard to keeping the membrane filter from fouling with solids and fibrous debris.
An advance in MBR design occurred in 1989 with Yamamoto’s invention to insert the membrane directly into the bioreactor. With the membrane unit submerged, aeration can be used to minimize membrane fouling, keep the solids in suspension and provide oxygen to the biomass. Both flat and hollow fiber membranes are used for this purpose.
The main challenge in operating an MBR system is to control fouling of the membrane. Fouling increases with filtration time resulting in a reduction in flux and an increase in the trans-membrane pressure differential. Fouling issues require that either the driving pressure be increased to maintain permeate flow or the membrane module be taken out of service for cleaning. This increases energy consumption and operating expense, respectively.
Periodic membrane cleaning is accomplished by various procedures specific to the foulant characteristics and membrane configuration, but generally consists of backflushing the membrane to physically remove the foulant in conjunction with chemical cleaning with sodium hypochlorite and citric acid.
The ability of MBR technology to remove BOD, COD and nutrients is well documented. Reductions of BOD and COD concentrations of 98% are achievable as compared to 85% to 95% by conventional activate sludge bioreactors. Proper incorporation of MBR technology into the wastewater treatment process reduces nutrients levels such as ammonia nitrogen by 99% and phosphorous by 96%. These results may vary, however, depending on the characteristics of the waste stream, overall treatment process and plant operating conditions.
MEMBRANES AND WASTEWATER RECLAMATION
Industrial plants that wish to use reclaimed wastewater for utility and process applications must begin by assessing the source water quality. In many cases, the tertiary municipal wastewater can be used without further treatment such as in irrigation and some course washing and rinsing processes. However, other higher level applications require further treatment to reduce total dissolved solids, organics, bacteria and viruses. This high quality wastewater is also acceptable for use as cooling tower makeup, boiler feedwater and selected process applications.
The main challenge in using a membrane process like reverse osmosis (RO) to treat tertiary wastewater is fouling control. Fouling of RO membranes arises due to the accumulation of colloidal material, mineral scale deposits, bacteria growth, and organics on the membrane surface and in the water flow channels. This dramatically shortens the run time between chemical cleanings and increases operating costs.
Pretreating the RO feedwater by microfiltration (MF) or ultrafiltration (UF) greatly reduces the fouling rate of the RO membranes due to colloids and bacteria. Microfiltration removes particulate matter down to the 0.1 to 10 micron range. This is within the bacteria-removal range. Ultrafiltration membranes have a finer pore size gradient, which effectively removes solids within the 0.001 to 0.1 range. This is in the realm of virus removal.
To further protect RO membranes from biological foulants, chlorine is added ahead of the MF/UF membranes and again ahead of the RO membranes. Although membranes have a low tolerance for free chlorine, the use of chloramines (combined chlorine) is an acceptable practice. A continuous dosage of 1 to 2 ppm chloramine is effective in controlling bacteria populations on the membrane surfaces.
Because tertiary wastewater contains high levels of calcium hardness, alkalinity and phosphate, fouling of the RO membranes with calcium phosphate and calcium carbonate is a potential problem. Scale deposits that accumulate on the membrane surface and in the flow channels results in a decrease in flux and an increase in pressure differential across the array. This condition is best controlled by reducing the percent recovery, adjustment of the feedwater pH and judicious application of chemical antiscalants. Specific recommendations for the pretreatment of the RO feedwater to minimize scaling is best done after a detailed analysis of the RO feedwater chemistry.
Membrane manufacturers have responded to the growing market for low-fouling RO modules that are tolerant of higher organic loadings and other foulants commonly found in municipal and industrial wastewater. These membranes claim to offer stable flux and salt rejection characteristics when used in these more-challenging applications. Pilot studies and full-scale operations with conventional polyamide and low-fouling composite membranes have demonstrated success with extending the intervals between membrane cleanings to 6 months or more.
USING WASTEWATER AS COOLING TOWER MAKEUP
The purpose of a cooling tower is to conserve water. By rejecting unwanted heat to the atmosphere via evaporative cooling, the water can be recycled several times prior to discharge to waste.
Traditionally, cooling tower makeup is obtained from fresh, potable water sources. However, this is changing as the competition for freshwater supplies increases. In some cases, municipalities have given large industrial consumers little option but to use treated wastewater for non-potable requirements.
Can treated municipal or industrial wastewater be reused as cooling tower makeup? Absolutely, as demonstrated by many successful applications in the pulp and paper, steel, auto, power and other manufacturing industries.
Potential sources of cooling tower makeup must be evaluated to determine their suitability for cooling tower makeup. In particular, wastewater presents a few challenges with regard to:
- Dissolved and suspended solids (total solids)
- Microorganisms
- Organics
- Nitrogen-ammonia
- Phosphate
- Heavy metals
Although chemical analyses can quantify the concentrations of each of these impurities, wastewater supplies tend to vary in quality over time. Therefore, several samples should be analyzed over an extended time period to assess the typical and maximum concentration levels.
The cooling tower system must be evaluated to predict the impact treated wastewater will have on the system metallurgy. Deposition of calcium phosphate and calcium carbonate in heat exchangers and low-flow areas can retard heat transfer and promote underdeposit corrosion. Nitrogen-ammonia is aggressive to copper and yellow metal alloys. High chloride levels are known to promote pitting of stainless steels. The impact of pH and alkalinity should be reviewed with regard to the corrosion of galvanized steel. These and other potentialities must be included in the project feasibility workup to avoid unintended consequences of scale deposition, corrosion, microbiological growths and fouling.
Changing the source for cooling tower makeup may have an impact on the cycles of concentration (COC). COC is the ratio between the concentration of impurities in the makeup to the same impurity in the recirculated cooling water. A cooling tower with a makeup conductivity of 300 micromhos/cm and a corresponding cooling water conductivity of 900, for example, is operating at 3 cycles of concentration. Over concentration of makeup water dissolved solids can lead to deposit formation and corrosion in the heat exchangers, distribution piping and cooling tower. Operating at low cycles increases the makeup demand. Determining the optimum COC is critical in terms of maximizing efficiency and controlling cost.
With regard to the reuse of treated industrial wastewater, decreasing the COC to minimize scale and corrosion problems is not necessarily a negative situation as the wastewater was already being discharged to drain prior to the initiation of the reclaim project. The primary consideration should be the net decrease in fresh water withdrawals made possible by the reuse of wastewater.
Using wastewater as makeup often requires a modification to the traditional water treatment protocols commonly used with better quality fresh water sources. Scale control agents, corrosion inhibitors and biocides should be evaluated to determine their efficacy when using alternate makeup supplies. In general, the need for oxidizing biocides like chlorine or bromine may increase due to the higher concentration of organics and microbiological nutrients. However, the need for corrosion inhibitors like phosphate, may decrease since the wastewater already contains phosphate.
As indicated previously, membrane processes are available to improve the quality of plant wastewater following conventional activated sludge, membrane bioreactor (MBR), or other treatment methods. Microfiltration, ultrafiltration and reverse osmosis can substantially reduce the levels of calcium hardness, phosphate, alkalinity, chlorides, ammonia, bacteria and virus. Membrane treatment offers several advantages in that many of the troublesome impurities found in treated wastewater are eliminated ahead of the cooling tower. Therefore, the tower can operate at much higher cycles of concentration, which reduces the makeup demand and the consumption of water treatment chemicals. However, the capital cost for membrane separation equipment is significant, so a calculation of the return on investment should be made to assess the overall benefit of the various chemical and mechanical treatment options.
SUMMARY
Membrane separation technology can and is being used to effectively treat municipal and industrial wastewater for reuse as cooling tower makeup. MBR, MF, UF and RO processes eliminate or reduce impurities that might otherwise adversely affect cooling tower operations. This includes BOD, COD, phosphate, ammonia, chlorides, alkalinity, calcium, bacteria and virus; to name a few. Success requires a thorough evaluation of the system metallurgy, operating conditions, overall impact on freshwater withdrawals, and return on investment. These water conservation efforts will help extend fresh water supplies, minimize waste and meet the future demand for potable water by a burgeoning urban population.
Florida and California Reclaimed Water Quality
|
Parameter |
Orlando |
Tampa |
Los Angeles |
San Francisco |
|
Conductivity |
1200 to 1800 |
600 to 500 |
2000 to 2700 |
800 to 1200 |
|
Calcium Hardness |
180 to 200 |
100 to 120 |
260 to 450 |
50 to 180 |
|
Total Alkalinity |
150 to 200 |
60 to 100 |
140 to 280 |
30 to 120 |
|
Chloride |
20 to 40 |
30 to 80 |
250 to 350 |
40 to 200 |
|
Phosphate |
18 to 25 |
10 to 20 |
300 to 400 |
20 to 70 |
|
Ammonia |
10 to 15 |
5 to 15 |
4 to 20 |
2 to 8 |
|
Suspended Solids |
3 to 5 |
3 to 5 |
10 to 45 |
2 to 10 |
Source: Guidelines for Water Reuse, USEPA/625/R-04/108, September 2004, Office of Wastewater Management, Washington, DC
Water Technology Report 