Wastewater Treatment, Decay, Mass Balance and Water Use: 11-CS-3 Engineering Management, May 2018, Q4

Question 4: Wastewater Treatment, Decay, Mass Balance and Water Use (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Activated-Sludge Wastewater Treatment Flow Diagram

 Raw wastewater (influent)
      │
      ▼
[1 Screening & Grit removal] ── removes rags, debris, grit/sand
      │
      ▼
[2 Primary clarifier] ── settles suspended solids; removes ~30% BOD & most TSS
      │
      ▼
[3 Aeration tank (activated sludge)] ── microbes + air consume dissolved
      │        organic matter (removes most soluble BOD)          │
      ▼                                                           │ return
[4 Secondary clarifier] ── settles microbial biomass (sludge) ───┘ activated
      │        (removes biomass, remaining BOD/TSS)                  sludge (RAS)
      │  └─ waste activated sludge (WAS) → thickening/digestion/dewatering
      ▼
[5 Disinfection] ── chlorine / UV / ozone; inactivates pathogens
      │        (+ optional tertiary nutrient removal before this)
      ▼
   Treated effluent → receiving water

Screening/grit protects equipment; the primary clarifier removes settleable solids and some BOD; the aeration tank's micro-organisms consume the dissolved organic matter (the core BOD removal), and the secondary clarifier settles that biomass, recycling some (RAS) to sustain the culture and wasting the rest (WAS); disinfection then kills pathogens before discharge, with optional tertiary treatment for nutrients.

(b) Bacterial Decay Calculation

t = \frac{1}{k}\ln\!\left(\frac{N_0}{N}\right) = \frac{1}{3.4}\ln\!\left(\frac{10^{5}}{1}\right) = \frac{11.513}{3.4} \approx \boxed{3.39\ \text{days}}

(c) Selenium Mass Balance

Diagram (flows in m³/s, concentrations in mg/L):

 Creek 5.5 (0.0012)
      │
      ├──────────────► withdraw 1.0 to irrigation
      │                        │
 4.5 (0.0012) bypass          ├─ 0.5 to ground/plants (lost)
      │                        └─ 0.5 return runoff (1.0 mg/L Se)
      ▼                                    │
      └────────────── mix ◄────────────────┘
                 Downstream: Q = 4.5 + 0.5 = 5.0 m³/s

Steady-state selenium mass balance downstream (load in = load out; convert m³/s × mg/L × 1000 L/m³ = mg/s):

\dot m = (4.5)(0.0012)(1000) + (0.5)(1.0)(1000) = 5.4 + 500 = 505.4\ \text{mg/s}

C = \frac{\dot m}{Q} = \frac{505.4\ \text{mg/s}}{5.0\times1000\ \text{L/s}} \approx 0.101\ \text{mg/L} \approx \boxed{0.10\ \text{ppm}}

The steady-state selenium concentration in the stream is about 0.10 ppm (0.10 mg/L)—roughly 84 times the upstream background of 0.0012 mg/L, driven almost entirely by the concentrated irrigation run-off.

(d) Highest Water Use and Reduction Technologies

Agriculture (irrigation) is the highest use worldwide—about 70% of freshwater withdrawals. Two reduction technologies: drip (micro) irrigation, delivering water directly to roots and cutting evaporation/runoff; and water reuse/recycling of treated effluent or greywater (plus efficiency measures like low-flow fixtures and leak reduction).

(e) Definition (any one)

Aquifer: a permeable underground geologic formation that stores and readily transmits usable quantities of groundwater. (Or BOD: dissolved oxygen consumed by microbes decomposing organic matter—a measure of organic pollution. Or turbidity: cloudiness of water from suspended particles.)

Question 4 of 5: Wastewater Treatment, Decay, Mass Balance and Water Use