Why Your Wastewater Permit Demands Different Sample Types

Have you ever looked at your wastewater permit and thought, “Why must I collect this sample as a composite and this one as a grab when they’re from the same location?” Why does it matter, and what are the results really saying? 

In wastewater, we must prove our effluent quality. We must demonstrate that we are meeting discharge requirements set by the primacy agency—requirements designed to protect public and environmental health. In this endeavor, wastewater permits outline frequency of samples collected, sample locations and type of sample collection to produce data that represents the whole. 

Frequency provides more data and sample locations provide consistency, but why specify sample type? The answer is accuracy. 

Two essential sample types 

Wastewater permits can have various sample types, but two types appear on every permit: grab samples and composite samples. 

Grab samples consist of either a single discrete sample or individual samples collected over a period not exceeding 15 minutes. This means the sample collected is a snapshot of water quality at a very specific time. By this definition, it would seem that grab samples don’t represent effluent quality as a whole and therefore contradict the intention of sampling. 

Composite sampling occurs when multiple grab samples are taken over a period of time and combined into a single sample. Effectively taking many snapshots, this creates a more representative sample that more accurately reflects water quality. 

Using this rationale, a question emerges: Why have grab samples at all? Why not have all samples collected as composites so they’re as representative as possible? 

The holding time challenge 

There’s one main reason: holding times. 

Holding times are the maximum time allowed between sample collection and analysis. Certain samples degrade very quickly, and exceeding the holding time for a specific sample invalidates the accuracy of the results.  

For example, pH samples have a 15-minute holding time. The only way to collect and analyze this water quality parameter is through a grab sample. A composite sample would take longer than 15 minutes to collect. The other water quality parameter we see on wastewater permits as a grab sample is E. coli. 

On the other hand, composite samples work for samples that generally have a holding time greater than 48 hours. Most samples collected within a wastewater system are composite samples. 

The complexity of composite sampling 

However, composite samples are more nuanced than simply collecting multiple grab samples and combining them into a single sample. First, composite samples must have a minimum of four grab samples spread across a six-hour period, equally spaced every two hours with a minimum 100-milliliter sample collected. This ensures that, as treatment conditions change throughout the day, snapshots are collected to represent these changes. 

The caveat here is that these samples must be proportionate to flow. As flows change throughout the day, the proportion of the composite sample must represent this change.  

A simple example: At 8 a.m. you collect 0.5 liters of sample when flow is 10,000 gallons. At 10 a.m. you collect another 0.5 liters of sample, but the flow is 20,000 gallons. Your composite sample should consist of 0.25 liters from the 8 a.m. sample and 0.5 liters from the 10 a.m. sample to account for the change in flow. 

The gold standard: 24-hour sampling 

You may see another type of composite sampling: the 24-hour composite. These, too, are flow proportionate and considered the most representative because samples are taken throughout the entire day, including at night when flows are often at their lowest.  

These samples are best collected using an autosampler which, when set up properly, can gather samples at scheduled flow intervals. To be set up as flow proportionate, the samplers must have telemetry connected to a flow meter. 

The sample collected here will be most representative of the whole for that day. Done throughout a week or month, it can provide accurate data to prove effluent water quality. 

The requirements in your permit reflect careful scientific consideration. Each sampling method serves its purpose: grab samples capture parameters that degrade quickly, while composite samples provide comprehensive pictures of stable parameters. Understanding these distinctions helps operators collect meaningful data that truly represents their facility’s performance and ensures continued protection of public health and our environment. 

This article is funded by EPA under RCAP’s Treatment Works 2  2023-2026 grant.