11.2 Water Supply and Wastewater Treatment

10.2 Water Supply and Wastewater Treatment

1. Drinking Water Quality Standards

The regulatory framework for drinking water safety in the United States is established by the Safe Drinking Water Act (SDWA). The EPA sets standards that apply to public water systems, categorized into primary and secondary regulations.

• National Primary Drinking Water Regulations (NPDWRs): Enforceable standards that limit contaminant levels to protect public health. For each regulated contaminant, the EPA establishes a health-based Maximum Contaminant Level Goal (MCLG)—a non-enforceable concentration at which no adverse health effects are anticipated (often set to zero for carcinogens). The EPA then sets an enforceable Maximum Contaminant Level (MCL) as close to the MCLG as feasible using the best available technology and considering economic feasibility. When a contaminant cannot be easily measured, the EPA establishes a Treatment Technique (TT), which is a mandatory treatment process (e.g., filtration or corrosion control).
• National Secondary Drinking Water Regulations (NSDWRs): Non-enforceable guidelines for contaminants that affect the aesthetic, cosmetic, or technical properties of drinking water. These guidelines cover parameters such as taste, odor, color, corrosivity, and Total Dissolved Solids (TDS) (recommended limit of 500 mg/L) or pH (recommended range of 6.5 to 8.5).

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2. Drinking Water Treatment Processes

Conventional water treatment plants utilize a sequence of physical and chemical barriers to remove pathogens, suspended solids, and chemical contaminants from surface water sources.

Coagulation

Natural waters contain suspended solids, including very fine colloidal particles (under 1 micron) that carry negative electrostatic surface charges. Because these negative charges repel each other, the particles remain in suspension indefinitely. Coagulation is the chemical process of destabilizing these colloidal charges. Coagulants such as aluminum sulfate (alum, $Al_2(SO_4)_3$) or ferric chloride ($FeCl_3$) are added to the water. This is followed by rapid mixing (velocity gradient $G > 700 \text{ s}^{-1}$ for less than 30 seconds) to ensure that the chemical reactions occur uniformly and instantaneously, neutralizing the negative surface charges.

Flocculation

Flocculation is the physical process of agglomerating the destabilized particles into larger, visible masses called floc. It requires gentle, slow mixing (velocity gradient $G$ from 10 to 70 $\text{ s}^{-1}$ for 20 to 30 minutes) to promote particle collisions while preventing the fragile flocs from being sheared apart by turbulent forces.

Sedimentation

Sedimentation is a gravity-settling process where the aggregated flocs are removed in a clarifier basin. Clarifier design is based on three critical parameters:

1. Detention Time ($t_d$): The average time water remains in the basin: $t_d = \frac{V}{Q}$ Where $V$ is the basin volume and $Q$ is the flow rate.
2. Surface Overflow Rate (SOR): The hydraulic loading per unit of surface area: $v_0 = \frac{Q}{A_s}$ Where $A_s$ is the surface area of the basin. Particles with a settling velocity $v_s \ge v_0$ are completely removed. Particles with $v_s < v_0$ are partially removed in proportion to the ratio $v_s/v_0$.
3. Weir Loading Rate (WLR): The flow rate per unit length of effluent weir: $WLR = \frac{Q}{L_w}$ Where $L_w$ is the total length of the effluent weir. A low WLR prevents high-velocity currents near the outlet from pulling settled flocs over the weir.

Filtration

Following sedimentation, water passes through a granular media filter to remove remaining suspended particles.

• Rapid Sand Filter: The standard system, typically consisting of a bed of sand and anthracite coal (dual-media). The typical filtration rate ranges from 2 to 10 gpm/ft² (5 to 25 m/h).
• Backwashing: When the filter media becomes clogged, headloss increases. The filter is taken offline and backwashed by pumping treated water upward through the bed at a high velocity. This fluidizes and expands the media bed by 20% to 50%, releasing trapped solids which are carried away in washwater troughs.

Disinfection

Disinfection destroys pathogens in the water before distribution.

• Chlorine Chemistry: When chlorine gas ($Cl_2$) or hypochlorite salt is added to water, it hydrolyzes to form hypochlorous acid ($HOCl$), which partially dissociates into hypochlorite ion ($OCl^-$): $Cl_2 + H_2O \rightleftharpoons HOCl + H^+ + Cl^-$ $HOCl \rightleftharpoons H^+ + OCl^-$ $HOCl$ is the most effective disinfectant, being 80 to 100 times more potent than $OCl^-$. Because the dissociation is pH-dependent ($pK_a \approx 7.5$ at 25°C), disinfection is highly efficient at lower pH values (pH < 7.5) where $HOCl$ remains dominant.
• Free Chlorine vs. Combined Chlorine: Free chlorine refers to the concentration of $HOCl$ and $OCl^-$. If ammonia ($NH_3$) is present, chlorine reacts with it to form chloramines, known as combined chlorine. Combined chlorine is a weaker disinfectant but provides a stable, long-lasting residual in the distribution system.
• CT Concept: Disinfection efficiency is determined by the product $C \cdot t$, where $C$ is the disinfectant concentration (mg/L) and $t$ is the contact time (minutes). For design, $t_{10}$—the time required for 10% of the water to pass through the basin—is used to account for short-circuiting. The EPA provides tables of required CT values for specific log-inactivations (e.g., 3-log or 99.9% reduction of Giardia lamblia) under varying pH and temperature conditions.

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3. Wastewater Collection and Sewer Design

Wastewater collection networks consist of gravity sewers designed to flow as open channels (partially full, not under pressure).

• Manning's Equation: Used to calculate the flow velocity ($V$): $V = \frac{1.486}{n} \cdot R^{2/3} \cdot S^{1/2}$ Where $n$ is the roughness coefficient (typically 0.013 for concrete pipes), $S$ is the pipe slope (ft/ft), and $R$ is the hydraulic radius (ft), defined as the wet cross-sectional area ($A$) divided by the wetted perimeter ($P$): $R = \frac{A}{P}$
• Design Constraints:
  • Self-Cleansing Velocity: To prevent the deposition of organic solids and sand, sewers must maintain a minimum velocity of at least 2.0 ft/s (0.6 m/s) when flowing full or half-full.
  • Maximum Velocity: To prevent abrasive wear on the pipe material, velocities should not exceed 10 ft/s (3.0 m/s).
  • Partial Flow Hydraulics: Sewers are typically designed to flow at 50% to 80% of full depth at peak design flow to ensure adequate ventilation space above the liquid level and prevent pressure buildup.

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4. Wastewater Treatment Processes

Wastewater treatment is governed by the Clean Water Act (CWA) and enforced through the National Pollutant Discharge Elimination System (NPDES) permit program. Municipal treatment is divided into levels:

1. Preliminary Treatment: Screens and grit chambers remove large debris, rags, and sand to protect downstream equipment.
2. Primary Treatment: Physical settling in primary clarifiers to remove settleable organic solids (typically removes 50-60% of suspended solids and 30-40% of biochemical oxygen demand, BOD).
3. Secondary Treatment: Biological processes designed to remove dissolved organic matter (measured as $BOD_5$) and remaining suspended solids.

Suspended Growth: Activated Sludge Process

The activated sludge process utilizes a suspended growth system where a high concentration of microorganisms (mixed liquor) is kept in suspension in an aeration tank.

• Process Layout: Influent enters the aeration tank where oxygen is supplied. The mixture flows to a secondary clarifier. The settled biomass is split: some is returned to the aeration tank as return activated sludge (RAS) to maintain biomass levels, and the excess is wasted as waste activated sludge (WAS) to control the sludge age.
• Key Design Parameters:
  • Mixed Liquor Volatile Suspended Solids (MLVSS): A surrogate measure of the active biological concentration ($X$, in mg/L) in the aeration tank.
  • Food-to-Microorganism (F/M) Ratio: Represents the organic loading rate on the biomass: $F/M = \frac{Q \cdot S_0}{V \cdot X}$ Where $Q$ is influent flow rate, $S_0$ is influent $BOD_5$ concentration, $V$ is aeration tank volume, and $X$ is MLVSS.
  • Mean Cell Residence Time (MCRT): The average time a microorganism spends in the system (also known as solids retention time, SRT): $\theta_c = \frac{V \cdot X}{Q_w \cdot X_w + Q_e \cdot X_e}$ Where $Q_w$ and $X_w$ are the flow rate and concentration of the WAS, and $Q_e$ and $X_e$ are the flow rate and concentration of the effluent.
  • Sludge Volume Index (SVI): The volume in mL occupied by 1 gram of activated sludge after settling for 30 minutes in a 1-liter graduated cylinder. It indicates the settling characteristics of the sludge: $SVI = \frac{\text{Settled Sludge Volume (mL/L)} \cdot 1000}{\text{MLSS (mg/L)}}$ An SVI of 50 to 150 mL/g indicates good settling. An SVI above 150 mL/g indicates sludge bulking, which can lead to solids escaping in the effluent.

Attached Growth: Trickling Filters

In trickling filters, microorganisms are attached to a fixed media (such as stones, gravel, or plastic packing). Wastewater is sprayed over the top and trickles down, allowing the biofilm to absorb and degrade the organic matter.

• Sloughing: As the biofilm grows thicker, it becomes anaerobic at the media surface and eventually detaches. This process is called sloughing, and the sloughed solids are removed in a secondary clarifier.