Four Filters and a Peak-Day Demand
A 12-MGD conventional surface water plant has six rapid-sand filters, each with 400 ft² of surface area, designed for 4 gpm/ft². A storm pushes raw turbidity from 6 to 40 NTU; filter runs fall from 60 hours to 22 hours and finished water sits at 0.14 NTU. On Monday two filters come out of service for media replacement, a job scheduled months ago. It is the third week of July, the system is at peak demand, and the superintendent says the plant "has to stay at 12 MGD." The junior operator proposes raising the alum dose, backwashing more often, and running the remaining filters harder to keep up.
1. Can the plant make 12 MGD on four filters? Show the loading calculation, state the maximum flow, and give your recommendation.
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Convert flow to gpm: 12 MGD x 694 gpm/MGD = 8,330 gpm.
Available filter area: 4 x 400 ft² = 1,600 ft².
Loading rate: 8,330 / 1,600 = 5.2 gpm/ft² against a design of 4.0 - a 30% overload. Overloading does not just shorten runs; it drives media disturbance, turbidity breakthrough, and sheared floc into the clearwell.
Maximum at design rate: 4.0 x 1,600 = 6,400 gpm = 9.2 MGD. Even with only one filter out (2,000 ft²), the ceiling is 8,000 gpm = 11.5 MGD.
Recommendation: Do not attempt 12 MGD with two filters down. Options ranked: (1) stage the media job one filter at a time, which caps the plant near 11.5 MGD and is still short of demand; (2) draw down storage and issue a voluntary conservation notice; (3) postpone the media work until after peak week. The judgement call is that a filter run beyond design rate is a compliance and public health risk, not just an efficiency loss - the superintendent's demand target does not override the loading limit.
2. Raw turbidity is high and you may need more coagulant. What would you change first, and what evidence would tell you within a shift whether the change is working?
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Do not simply raise the alum setpoint. Alum consumes alkalinity and depresses pH; over-dosing depresses pH below the coagulation optimum and makes things worse while also raising finished-water corrosivity. Sequence:
- Verify the feed is real - compare actual dose to setpoint (drawdown or weigh scale), confirm pump calibration and that the day tank is the concentration you think it is.
- Run a jar test on the current raw water at two or three doses and two pH values; the storm changed the particle character, so last month's optimum is not this week's.
- Check pH and alkalinity after rapid mix, and check rapid-mix and flocculator energy - short-circuiting or a broken flocculator paddle can look exactly like under-dosing.
- Consider a polymer as a filter aid versus a coagulant aid; these solve different problems (run length versus settled-water clarity).
Leading indicator: settled-water turbidity. If settled turbidity drops below roughly 1 to 2 NTU, the filters get a light load and runs lengthen; if settled turbidity is unchanged while you raise alum, the problem is upstream of the filters.
Cost check on more backwashing: a backwash at 15 gpm/ft² on a 400 ft² filter is 6,000 gpm for about 10 minutes, roughly 60,000 gallons of finished water per wash. At peak demand, that water is being spent, and backwashing a filter that is not dirty only re-suspends solids and spikes turbidity. Backwash on headloss or effluent quality, not on a schedule.
3. During the media job one filter's individual turbidimeter reads 0.6 NTU while combined filter effluent is 0.14 NTU. What do you do, and how does that reading fit the regulatory framework?
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Regulatory frame: combined filter effluent must be 0.3 NTU or less in at least 95% of monthly measurements and must never exceed 1.0 NTU. Individual filter effluent triggers are separate - an IFE above 1.0 NTU on two consecutive 15-minute readings requires reporting and follow-up, and an IFE above 0.5 NTU for two consecutive months in the same filter requires a self-assessment. A single 0.6 reading is above the 0.5 threshold but below the 1.0 immediate trigger.
Actions in order:
- Confirm the reading - grab sample against a second instrument, and check the turbidimeter's calibration and sample line for air bubbles or fouling. A plugged sample line reads high or erratic.
- Walk the filter - check for media disturbance from the adjacent work, air binding, a partially closed effluent valve, or a filter-to-waste valve left open.
- If confirmed, take that filter out of service or place it on filter-to-waste, backwash, and inspect the media surface before returning it.
- Document readings, actions, and times, including the parallel media project. Regulators look at trends, and a spike tied to construction is a defendable record while no record is not.
The judgement point is that the combined effluent number can look fine while one filter is degrading - 0.14 NTU blended does not mean the 0.6 filter is harmless, because media defects worsen quickly.