Does rainfall drive WWTP sludge pumping?

From the QuickBooks ledger, June 2025 through March 2026: 29 sludge-removal transactions totaling roughly $107,000. Of those, 16 are categorized “emergency” and 4 “routine.” Emergency rate ≈ $323/kgal; routine rate ≈ $194/kgal. The FY 25-26 adopted sewer budget assumed $5,500 for the full year; the projected actual is $60,837 — a 10× overrun, with a mid-year amendment of $5K already approved.

The operator’s memos on the emergency invoices repeatedly describe the pumps as “EQ tank pump-down” or “sewer plant pump” — language consistent with drawing down the equalization tank when the plant can’t process inflow fast enough. If that’s the mechanism, the trigger should be antecedent rainfall.

Tempting, but the dates don’t support it. The dates we have for pump-outs are QuickBooks invoice dates, not the dates the pump actually ran. Where a memo records the real event date, it often differs from the invoice date by days to a month, and several invoices bundle two or three separate pumps under one date. So lining pump-out “dates” up against rainfall mostly measures the vendor’s billing cycle, not hydraulics.

For reference, the chart below compares antecedent rainfall on invoice dates against all other days. It does show pump-out invoices landing on drier-than-typical dates — but because of the billing-date problem we treat that as suggestive at most, not as evidence about when the plant was actually under load.

Rather than lean on unreliable event dates, the rest of this page asks a question that needs no pump-out dates at all: does the plant’s own flow show it running out of capacity when it rains? If it does, the pump-outs have an obvious structural explanation regardless of exactly when each one was billed.

At the Village WWTP, yes — weakly but coherently. The heatmap below tests every (start, end) cumulative rainfall window between days[d − start] and day[d − end] and colors each cell by its Pearson correlation with flow on day d. The right grid (Village WWTP) shows a contiguous warm block: neighboring windows agree with each other, which is what a real — if shallow — signal looks like. Flow stays elevated for a week or more after a big storm, the slow build consistent with groundwater and inflow leaking into the collection system. The left grid (Red Hook Commons) is salt-and-pepper — isolated warm cells next to cool ones, the texture of noise. That’s an artifact of measurement resolution, not hydraulics: Commons flow is reported to the nearest 0.001 MGD on a base of about 0.007 MGD, too coarse for a correlation to resolve.

So there isa rainfall signal at the Village plant. The question that matters for the pump-outs isn’t whether it’s strong — it’s what happens to that signal at the top of the range.

This is the key chart. We sort every day by how much rain fell over the Village plant’s best rainfall window, split the days into five equal-count buckets from driest to wettest, and plot the distribution of daily flow in each bucket — the median, the 75th percentile, and the 90th percentile.

If the plant had spare capacity, all three lines would rise together as it gets wetter. Instead, the median rises (the plant takes on more load with rain) while the upper percentiles flatten onto a ceiling right at the 0.050 MGD SPDES design line. More rain stops producing more flow — because the plant can’t pass more. The excess goes to the equalization tank, and the EQ tank is what gets pumped out. This is the statistical fingerprint of a censoredmeasurement: you aren’t seeing the plant’s true wet-weather load, you’re seeing it clipped at the plant’s real throughput.

The ceiling is robust: it sits at the same ~0.055 MGD whether we measure rain over 3 days or 15, and it appears in all four seasons (most sharply in winter, when frozen ground and snowmelt push the most groundwater into the system) — so it isn’t a wet-season coincidence. It is the behavior of a plant that is hydraulically undersized for its collection system’s wet-weather inflow.

For context, the histogram below shows every day of measured flow against each plant’s SPDES design capacity (0.050 MGD at the Village WWTP via sub-outfall 01A; 0.025 MGD at Red Hook Commons via sub-outfall 01B). Note the caveat the censoring analysis adds: because wet-weather flow is clipped at the design line, the count of days at or beyond capacity understates how often the plant would have exceeded it without the pump-outs holding it back.

Put together: rainfall raises load at the Village plant, the plant hits a hard ceiling at its design flow on wet days, and the EQ-tank pump-outs are the most natural explanation for what keeps it there. The 10× pump-out overrun reads less like discretionary spending and more like the recurring cost of moving wet-weather inflow the plant can’t process — an inflow & infiltration problem.

The DEC monthly DMR forms include a column for the operator’s gauge readings of daily rainfall, and it would be natural to use that column for any rainfall analysis. We don’t — and the reason is worth flagging on its own. Cross-checked against NOAA observations from a CoCoRaHS station 1.4 miles from the village, the operator-reported series is unreliable: 11% lower in total rainfall, ~5% of days disagree by more than half an inch, and many days have a major storm recorded by NOAA that the operator’s column shows as 0.00″.

Every rainfall chart on this page therefore uses NOAA as the source of truth, falling back to the operator’s column only where NOAA didn’t observe. A side effect of this check: the operator’s rainfall column on the DMR is itself unreliable enough to be of concern to NYSDEC, independent of the pump-out cost question.

The flow data points to wet-weather hydraulic load as the leading explanation: the Village plant runs into its design ceiling when it rains, and the EQ-tank pump-outs are the relief mechanism. The published records can’t prove that for any individualhaul — the QuickBooks memos describe pumps but not causes, the dates are billing dates, and we can’t reliably split events between the Village WWTP and Red Hook Commons. So these other contributors can’t be ruled in or out, and may stack on top of the capacity problem:

• Inflow & infiltration (leading) — the censored flow ceiling is the direct evidence: stormwater and groundwater entering the collection system faster than the plant can process, forcing diversion to the EQ tank. This is a collection-system/capital issue, not an operating choice.
• Equipment failures or maintenance backlog — pump-outs cluster in time (4 events in August, 4 in December, 4 in January), which could also fit an equipment-issue pattern layered on the baseline load.
• Biological process upsets — sudden MLSS drops or WAS-rate changes around event clusters would point to the activated-sludge tank failing, with the EQ tank used as a hydraulic buffer to keep influent off the bug population.
• Base-load drift — village sewer base flow may be growing fast enough to fill the EQ tank more often, compounding the wet-weather load.

To answer “what is driving the pump-outs” with the rigor the budget overrun warrants, the village would need to publish, per pump-out event:

• The plant pumped (Village WWTP / Commons / both).
• The operator’s stated cause (process upset, equipment failure, scheduled maintenance, hydraulic load).
• The EQ tank level at the time of the call and after the pump.
• The volume actually hauled (vs the volume billed, since three of the larger emergency invoices use rate-divided volume rather than measured volume).

None of this requires special equipment. It does require the operator to keep a log and the village to publish it.