Lecture 8 - Point Source Pollution

Point Sources of Pollutants

• Definition:
  • A substance becomes a pollutant when the receiving water's capacity to degrade, disperse, or dilute it is exceeded.
  • Exceeded means dilution, dispersion, or degradation cannot prevent harm to the aquatic ecosystem or legitimate (human) uses.
  • This definition implies that discharge is permissible as long as the processes of dilution, degradation, and dispersion protect the ecosystem and human uses. The primary focus is on dilution potential.
• Key Aspects:
  • Identifiable Source: Must be able to identify the source of entry into the receiving water.
  • Permitting: The ability to permit the entry, enabling treatment.
  • Treatment: Permits drive treatment and investment in treatment.
  • Monitoring: The site is monitored for compliance.

Diffuse vs. Micro Point Sources

• Diffuse Sources: Classically, diffuse sources are not manageable in the same way due to the lack of an identifiable entry point.
• Micro Point Sources: Small point sources, like septic tank systems, often lack identifiable discharge points, hindering permitting, treatment, and monitoring.

Wastewater Sources

• Domestic Wastewater Treatment Works: Traditional focus, exemplified by the Lancaster wastewater treatment works.
• Industrial Sources: Increasingly important to consider.
  • Examples: Discharges from industrial complexes, textile industries (detergents causing foam), and mixed industrial/domestic waste in rivers like the Ganges.

Industrial Effluent Categories

• Heavy Industries: Wastewater from heavy industrial processes.
• Light Industrial Work: E.g., the textile industry.
• Food Industry: E.g., breweries, milk/cheese processing plants generating wastewaters high in glutens.

Management of Industrial Discharges

• On-site Treatment: Industrial sites treat their wastewater and comply with permits.
• Trade Effluent to Sewer: Industrial wastewater sent to municipal wastewater treatment plants (e.g., Lancaster), with the utility charging the industrial sites for treatment.

Permitting Framework

• Objective: To control discharges from point sources to prevent harm to the aquatic ecosystem and human uses.
• Legislative Basis:
  • Legislation (e.g., Urban Wastewater Treatment Directive) sets minimum baselines.
  • The Environment Agency can set permits for substances not in legislation or impose stricter limits.

Approaches to Permitting

• Uniform Approach:
  • Based on size (person equivalent) or type of discharge.
  • Problem: Does not account for the receiving water's capacity to handle the wastewater.
• Catchment Based Approach:
  • Sets a permit for the entire catchment (e.g., total phosphorus discharge).
  • Problem: Does not account for the specific location of individual point sources.
• Site Based Permitting:
  • Informed by local conditions and the capacity of the receiving water.
  • Administrative burden but protects local differences.

Critical Load

• Definition: The threshold at which a receiving water can no longer handle the pollutant load from a point source without causing damage.

Critical Load Management: Mass Balance

• Conceptualization:
  • Considers a river reach with a point source discharge.
  • Accounts for loads from upstream tributaries, diffuse sources, subsurface (groundwater) discharges, and losses/gains within the reach.
  • The load is concentration times discharge ($Load = Concentration Discharge$).

Dilution Modeling

• Simplification of Mass Balance:
  • Focuses on upstream load, effluent load, losses/gains, and downstream load.
  • Equation: $Q<em>uC</em>u + Q<em>eC</em>e + L = Q<em>dC</em>d$
    • Where:
      • $Q$ = discharge
      • $C$ = concentration
      • $u$ = upstream
      • $e$ = effluent
      • $d$ = downstream
      • $L$ = losses or gains
  • Units must be consistent.

Mass Balance Principle

• Rate of change of mass is determined by what comes in minus what leaves.
• Equation: $\frac{\Delta (Q<em>dC</em>d)}{\Delta t} = Q<em>uC</em>u + Q<em>eC</em>e - Q<em>dC</em>d$

Simplifications for Permitting

• No Loss or Accumulation: The substance is conservative (L = 0).
• Perfect Mixing: Effluent mixes perfectly with the upstream tributary at the point of contact.
• No Change of Mixed Mass Through Time: The rate of change of mass is zero.

Conservative Dilution

• Equation: $Q<em>uC</em>u + Q<em>eC</em>e - Q<em>dC</em>d = 0$
• What goes into the reach is what leaves.

Rearranging the Equation

• To estimate downstream concentration ($C_d$):
  • $C<em>d = \frac{Q</em>uC<em>u + Q</em>eC<em>e}{Q</em>d}$
  • Where $Q<em>d = Q</em>u + Q_e$

Data Requirements

• Effluent Concentration: From consent or estimate.
• Effluent Discharge: Estimated from dry weather flow.
• Upstream Discharge: Monitoring data.
• Upstream Concentration: Monitoring data or Water Framework Directive class boundaries.

Limitations of Conservative Dilution

• Instantaneous Values: Only works for specific values of $Q<em>e$, $C</em>e$, $Q<em>u$, and $C</em>u$.
• Statistical Descriptors: Permits are based on statistical descriptors of a distribution (e.g., 90th percentile).
• Not Robust for Percentiles: Standard conservative dilution is not robust when using 90th percentiles.

Combined Distribution Approach

• Uses a distribution of values for inputs (QU, CU, QE, CE) to generate a distribution of values for the downstream concentration.
• Enables calculation of percentile BOD or DO concentrations to assess compliance with WFD classes.
• Can be implemented using Monte Carlo simulation.

Permit Components

• 90th Percentile or Mean: For total P and total N.
• MAC (Maximum Allowable Concentration): An upper-tier limit, often set as double the 90th percentile.
• Two Modeling Frameworks: Simple conservative dilution and combined distribution.

Wastewater Composition

• Predominantly Water: 99.9% by mass.
• Solids: 0.1%, mostly organic compounds (fats, carbohydrates, proteins).
• Inorganic Fraction: Grits, metal, salt, glass, etc.

Pollutant Concentrations in Raw Wastewater

• BOD: ~300 mg/L (limit under Urban Waste Water Treatment Directive is 25 mg/L).
• COD: ~400 mg/L (limit under Urban Waste Water Treatment Directive is 125 mg/L).
• Total N: ~40 mg/L
• Phosphorus: ~10 mg/L
• Stoichiometry: High N and P relative to C.

Emerging Contaminants

• Endocrine Disruptors:
  • Interfere with the endocrine system (hormone secretion).
  • Examples: Estrogen, pesticides, dioxins, PCBs.
  • Limited evidence on environmental quality standards and effects.
  • Example: Male fish developing deformities when exposed to synthetic estrogen.
• Pharmaceuticals and Personal Care Products (PPCPs):
  • Examples: Caffeine, ibuprofen, diazepam, prescription drugs.
  • Potential effects on ecosystems.
• Illicit Drugs: Monitoring in wastewater networks.
• Microplastics: Another emerging concern.