Wastewater treatment (part 1)

Activity: How effective are WWTPs?

Timing: Allow 15 minutes

You can now explore the scientific data that demonstrates the effectiveness of WWTPs. Study the following table and answer the questions below.

Levels of the sul1 AMR gene in influent and effluent at different WWTPs (Mosaka et al., 2023)
Genes WWTP Concentration (copies/ml) in the influent Concentration (copies/ml) in the effluent References
sul1 WWTP1 3 × 107 5 × 105 Mao et al., 2015
WWTP2 9 × 106 6 × 105 Mao et al., 2015
WWTP3 1.19 × 108 4.52 × 106 Zhang et al., 2017
  1. The sul1 gene confers resistance to sulfonamides. Does the treatment in the WWTP reduce or increase the concentrations of sul1?
  2. The removal efficiency of the treatment can be defined as:

  • where influent is the raw wastewater before treatment in a WWTP and effluent is the wastewater after treatment in a WWTP, emitted to the environment.

    Using the equation, calculate the removal efficiency for sul1 in WWTPs 1–3.

  1. Which of the WWTPs shows the most effective treatment?

Answer

  1. The treatment reduces the concentration of sul1 at all three WWTP.
  2. The removal efficiencies for each plant are:
    • WWTP1: efficiency = (3 × 107 – 5 × 105) / 3 × 107 = 2.95 × 107 / 3 × 107) = 0.98 or 98%
    • WWTP2: efficiency = (9 × 106 – 6 × 105) / 9 × 106 = 8.4 × 106 / 9 × 106) = 0.93 or 93%
    • WWTP3: efficiency = (1.19 × 108 – 4.52 × 106) / 1.19 × 108 = 1.14 × 108 / 1.19 × 108) = 0.96 or 96%

  3. The first plant is most effective, with 98% removal. However, while a large proportion of the resistance gene is removed, there is still considerable discharge of this resistance gene with the effluent.

    Removal efficiencies are often expressed on a log scale:

    • Removal efficiency (log scale) = log10 (concentration in influent / concentration in effluent)
  • So for WWTP1:
    • removal efficiency (log scale) = log10 (3 × 107 / 5 × 105) = 1.78
  • That is, removal is nearly 2 orders of magnitude.

When you’ve tried this activity you should return to Section 3.2 of the course [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

Calculating exposure in practice

Wastewater treatment (part 2)