wastewater-treatment plants, which are also called publicly owned treatment works (POTWs), for primary and secondary treatment. Primary treatment includes coarse debris screed, sand and grit removal and clarification. Secondary treatment includes attached-growth or suspendedgrowth biological treatment (using aeration), secondary clarification and disinfection. The primary and secondary treatments at municipal water-treatment plants remove most of the biochemical oxygen demand (BOD) and suspended solids. Typical secondary-treated-wastewater characteristics are pH 6 to 9, total suspended solids (TSS) of 35 mg/L, ammonia-N content of 35 mg/L, BOD of 25 mg/L, and chemical oxygen demand (COD) of 125 mg/L. As an example of industrial use, the typical requirements for power-plant cooling-tower makeup are pH 6.8 to 7.2 or 7.8 to 8.4, TSS 5 to 30 mg/L, ammonia-N 1–5 mg/L, and total phosphorus as phosphorus (TP–P) less than 1 to 5 mg/L depending on the cooling-tower cycles of concentration. The following is true for cooling tower makeup: • Ammonia increases corrosion rates of copper alloys; is a nutrient for microbes; and irreversibly reacts with chlorine • High suspended solids cause fouling • Phosphate (PO43–) causes scale formation with calcium; and is a nutrient for microbes, causing biofouling The tertiary treatment reduces the residual organic, ammonia, suspended solids and toxic chemical levels.
Guidelines for treatment levels The treatment level to be obtained is set by the 2012 U.S. Environmental Protection Agency (EPA) Guidelines for Water Reuse, state regulations, and cooling tower and demineralization makeup requirements. Table 1 lists the typical constituents of secondary treated water, and the possible tertiary treatment options. Along with the capacity, constituents, and constituent-removal requirements, CHEMICAL ENGINEERING
TABLE 1. WATER TREATMENT PROCESSES FOR TREATING SECONDARY TREATED WATER Constituents
Treatment
Ammonia
Biological treatment: Membrane bioreactors (MBR) Moving bed biofilm reactors (MBBR) Biological aerated filters (BAF) Trickling filters (TF)
Oil and grease BOD COD TOC TP-P TSS
Clarification, multimedia filtration, and microfiltration/ultrafiltration
Turbidity AI
Lime softening and pH adjustment
Fe, Mn
Aeration/chlorination, greensand filtration, lime softening, pH adjustment
Ca, Mg
Lime softening
SiO2
Lime softening, supplemental magnesium
the temperature and pH are very important in deciding the type of technology and the size of equipment. The sludge generated in the tertiary treatment is typically recycled to the same POTW that provided the secondary treated wastewater. The 2012 EPA Guidelines for Water Reuse present industrial water reuse guidelines in various U.S. states. Water quality requirements for industrial reuse are lower than the unrestricted urban-use criteria because of reduced contact potential with individuals. Bacteriological quality varies from 2.2 total coliform/100 mL to 200 fecal coliform or E. coli/100 mL. Carbonaceous BOD monthly averages are 15 to 30 mg/L and TSS limits are 5 to 45 mg/L. Nutrient removal is an
important step for industrial reuse, but it is not directly addressed in the EPA guidelines. To control these constituents, several biological wastewater-treatment options are discussed in this article, including MBR, MBBR and BAF.
MBRs, MBBRs and BAFs Membrane bioreactor (MBR). A common external side-stream membrane bioreactor is shown in Figure 1. The MBR system is a combination of an activated sludge bioreactor (suspended growth) and low-pressure membrane filtration. Secondary treated wastewater from a POTW enters a grid, is screened, and is then mixed with chemicals and activated sludge in an aerated bioreactor, followed by low-pressure acti-
TABLE 2. ADVANTAGES AND DISADVANTAGES OF THE MEMBRANE BIOREACTOR MBR Advantages
MBR Disadvantages
1. Eliminates solid / liquid separation and tertiary filtration; replaces the need for clarification, multi-media filtration and ultrafiltration 2. Great effluent water quality 3. Pretreatment for nanofiltration and reverse osmosis 4. Handles influent variability 5. Small footprint 6. Automation is relatively simple and reliable 7. Low hydraulic retention times
1. Additional membrane cleaning is required 2. Membrane maintenance 3. Fouling 4. High electric power requirement (air scouring) 5. Fairly new for tertiary application 6. High Opex 7. High Capex 8. Waste sludge can have a low settling rate
TABLE 3. ADVANTAGES AND DISADVANTAGES OF MOVING BED BIOFILM REACTORS (MBBR) MBBR Advantages
MBBR Disadvantages
1. Great effluent quality 2. Handles influent fluctuations 3. Small footprint 4. Lower power consumption 5. Increased biomass IFAS (integrated fixed-film activated sludge) mode 6. Stable under large load variations
1. Requires clarification and multimedia filtration 2. Ultrafiltration is needed for demineralization system 3. Media maintenance 4. High chemical costs 5. Has coarse bubble diffusers that have very poor oxygen-transfer efficiencies
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