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Small Business: How to Measure Industrial Discharge for Environmental Compliance

Jon Wachter

As a small business owner, you are called on to wear any number of hats each day:  sales evangelist, human resources counselor, financial wiz, marketing planner, and more! None of these jobs you’ve probably trained for, but all of which are vital to your company’s health and growth.

To add to the list, more and more small businesses are being required to measure, sample, and record their wastewater discharges to their local POTW (Publicly Owned Treatment Works).  Regulators do this for a number of reasons, including load balancing at the treatment plant, exclusion of potentially harmful chemicals in the sewer system, and compliance with the Clean Water Act.  While some POTWs provide their utility users with general guidance on where start and how to begin, others don’t – which to the overtaxed business owner is something of a headache!

Knowing Your Site Conditions

In general, a business owner needs to know:

  • Where their discharge line(s) are and how they are being fed from the plant?

  • Where and how do the discharge lines connect to the POTW’s sewer?

  • What the flow composition is for each line?

    • Clean, post-treatment industrial discharge

    • Untreated industrial discharge

    • Mixed industrial and sanitary discharge

  • Flow rate?

  • Frequency and schedule of discharge (i.e. continuous, batch, daily, etc.)?

  • Flow characteristics?

    • Temperature

    • Solids load

    • Known chemical contaminants

    • Suspected chemical contaminants

With good documentation, a business owner may know some of the above. But more likely they will need to bring in some professional assistance.

These can include:

The more complex the manufacturing process or the larger the business size, the more specialist assistance is usually required.

Submitting Your Plan

Once the necessary information has been gathered, a plan of action can be formed.  This plan of action, and the steps that the business is proposing, is then submitted to the regulating agency for their review and approval.  This review and approval process can be a back and forth dialog and may take several iterations to get to a plan that is approved by all parties.

For almost all business required to measure their wastewater discharge, a primary device will need to be installed, at which the business with measure their flow and/or sample.  Depending upon the agency, the relationship between the business and the agency, and the toxicity and treatment load of the discharge, the regulating agency may also periodically measure and/or sample concurrently with the business.

Measuring the Flow

Primary devices for industrial flow measurement and sampling are almost always flumes – although, rarely, they can be weirs (providing the flow stream composition is suitable).

Flumes are fixed, engineered devices that accelerate water in such a way as to develop a known relationship between the level (head) in the flume and the flow rate through the flume.

Weirs are engineered plates that are place perpendicular to the flow which develop a level-to-flow relationship by water spilling over the weir plate.

By allowing flow to go through it, a flume tends to collect fewer solids and less debris, making it easier to maintain and operate. Also, there are a wider range of flume styles and sizes than there are weirs – allowing a user to fine tune which flume they are using to their exact site / size / flow conditions.

Flumes, ranked in order of usefulness to industrial dischargers, include:

While there are other types of flumes (RBC and long-throated flumes) these are more useful for measuring agricultural flows than they are measuring industrial ones.

Above Grade Monitoring

For above-ground installations, the flume can be free-standing or it can be housed in a secured enclosure.

Palmer-Bowlus flume installed in a fiberglass enclosure for measuring industrial discharge.

Below Grade Monitoring

These conditions are somewhat rare and are most common in areas of the country where there is a little slope and where below-ground burial is impractical.

The more common method of primary device installation is in a below-grade manhole.  Depending upon the site and the existing infrastructure, it may be possible to repurpose a manhole already at the business.  More commonly, however, the requirements of the primary device are such that a new manhole needs to be installed.  When this happens, many regulators direct businesses to Packaged Metering Manholes.

Packaged Metering Manholes incorporate a primary device (usually a flume or weir), into a fiberglass manhole.

Factory incorporation means that:

  • The primary device is installed correctly – not always a given when trying to reuse an existing manhole

  • The unit is monolithic and watertight structure – the water measures is only that which is going through the line (no inflow through the manhole walls / top)

  • and that the unit is safe and corrosion resistant

Applying A Primary Device

When applying a primary device, it is important to remember that to operate properly certain conditions are necessary:

  • The flow entering the primary device cannot be turbulent or possessing excessive velocity. When those are present, some form of upstream flow conditioning – possibly a drop manhole or an Energy Absorbing one may be needed.

  • In general, the upstream slope must not exceed 2% – and should ideally be 0.5%

  • The upstream line shouldn’t have any bends, dips, junctions, or elbows, for 15 to 25 pipe diameters upstream of the primary device.

  • The primary device must be set level from front-to-back and from side-to-side

    • It should NOT match the slope of the line

These reasons are, in part, why many times it is not possible to use / repurpose an existing manhole.

Secondary Flow Meter

Depending upon the discharge permit, a business may only need to install a flow metering / sampling station and not do the actual flow metering / sampling themselves.  It that’s the case, their tale ends here.  If not, a secondary device (called a flow meter) is required.

A flow meter continuously monitors the level in the primary device and translates it into a corresponding flow rate. This flow rate is then totalized for later reporting.

Flow meters based upon ultrasonic level detection are the most common. They are low cost, are non-contacting (the sensor is not in the water), and require minimal maintenance.  Their accuracy is good and resolution is adequate for the application.

Most flow meters provide local indication of the level and both the instantaneous flow rate and the total flow rate.

It is this totalized flow that is usually reported on a monthly basis to the regulatory agency.

Flow Parameters

Sometimes the POTW wants more information that just the totalized flows.  Usually when they want additional information it is:

  • Temperature (i.e. commercial laundries)

  • Metals (i.e. metal finishers)

  • BOD (biological oxygen demand – food plants)

  • COD (chemical oxygen demand – chemical plants)

Monitoring of these parameters can – depending upon the flow meter – be done as part of the flume meter or may be monitoring by separate, dedicated instruments.

When monitoring secondary parameters, the measurements should be taken as close to the primary device’s point of measurement as practical to ensure that the readings are representative