Troubleshooting the mitigation of biological oxygen demand

Oct. 2, 2015

The problem was that biological oxygen demand (BOD) had to be mitigated using only physical chemical wastewater treatment.

Wastewater challenges can be similar for plants across process and even discrete-manufacturing industries. Procedures used to successfully troubleshoot aberrations in one industrial wastewater treatment plant are often relevant for a great many others. In today’s example, the problem was that biological oxygen demand (BOD) had to be mitigated using only physical chemical wastewater treatment. The facility in question has been generally able to discharge consistent violation-free wastewater.

However, for reasons then-unknown, the facility violated its discharge permit on calculated BOD lbs./day for two consecutive months, while self-monitoring.  This was very unusual, both from the perspective of the industrial company involved and that of its regulator.

There was also concern because the industrial company had just taken on a new industrial pretreatment coordinator.

If the repeat violations had continued, the industrial wastewater treatment discharger would have lost its self-monitoring sampling privileges.

That would have meant giving up its sampling schedule – i.e., a single 24-hour sampling period per month – and having the regulator conduct three continuous unscheduled 24-hour sampling periods per month, with sampling results subject to surcharges.

Violations background

As regards the two sample events that led to violations: if at that point self-monitoring hadn’t been being done, the first month’s violations would have led to surcharges of $7,000 and the second month an additional $3,000.

Some additional relevant data includes the following:

The plant operates at full production during the first shift, with limited production activity on the second and third shifts. This industrial company purchases approximately 20-million gallons of water per month and discharges approximately 700,000 gallons of treated wastewater into the Sewer Authority collection system each 24-hour production cycle.

The production process varies, with different and unique volumes from different production departments, and as to wastewater pH, contaminants and their concentrations.

The response to variation must be timely because the treatment plant has no equalization-tank capacity capable of "equaling out" the mixing variations. Therefore, a response during treatment is required.

The wastewater treatment facility operates 24 hours during full production days. Its three operators work eight-hour shifts. The first shift operator starts the wastewater treatment plant before production commences at about 7 am.

Flow before production commences is about 200 gallons per minute and during production averages 550 gallons per minute. Maximum is somewhere near 700 gallons per minute.

Just one catch

The facility has a catch basin to receive untreated wastewater from production. The basin is designed so floatable solids are scraped off of the top surface of the flowing channel and settle-able solids can settle in the channel bottom.

The wastewater flowing midstream in the Catch Basin channel is free of floatable solids and settle-able solids and will
consequently flow by gravity to the next process unit, which is a rotary screen to remove fine solids.

This wastewater leaving the rotary screen is gravity-flowed into the primary treatment portion of the wastewater treatment plant. Primary treatment includes an alkaline neutralization, adjusting pH by raising it to a constant minimum value above 8.0.

The next primary treatment involves acid-neutralization pH adjustment, done by lowering pH to a constant value below 5.5 and injecting an anionic polymer solution as the wastewater enters the clarification tank, i.e., a dissolved air flotation (DAF) unit.

The DAF unit is used for floatable solids removal and to clarify wastewater. The clarified wastewater is discharged from the bottom of the DAF Unit.

The wastewater discharge is alkaline neutralized by raising pH to a constant minimum value above 6.0 to satisfy the discharge permit limit.

Findings & actions

Examination over time revealed wastewater going to sewer was not as clean as the process units’ discharge, including that of the DAF Unit.

It was found some waste wasn’t related to the process units. Certain waste sources were bypassing the process units and gravity flowing directly into the Sewer Authority collection system.

Steps were taken to capture that untreated wastewater before it went to sewer and first subject it to treatment. It was later confirmed the changes were successful, solving the mystery of the first of the two continuous violations.

Sampling was done a second time with the correction in place. A violation persisted, caused re-examination of everything done the previous month. The second violation uncovered was due to a mechanical malfunction in plant production.

Management was made aware of the mechanical malfunction and the need to keep the mechanical equipment in acceptable running order.

Following mitigation of both conditions, a third sample event in the third consecutive month was free of any violation and therefore compliant.

How Wastewater Dan does it, step-by-step

A wastewater facility had consistent documented permit violations involving significant surcharges. How could this batch treatment facility deliver consistent compliant effluent?

The first step was to get the facility remove the facility from the discharge permit notices of violation.

Training was required throughout the facility. It became clear the treatment procedures were incorrect, as were the internal-quality control-lab results. It was a question of "where to start?"

Data gathered

Data was needed to analyze treatment procedure and performance.

Next was analysis of operator performance for four operators andinteractions resulting from shift change-overs.

The in-house quality-control lab procedures for analyzing treated samples had to be documented.

How wastewater treatment compared to the discharge-permit requirements, including surcharge calculations, had to be determined.

Of four primary discharge permit issues the first was pH, second was total suspended solids (TSS), third was a biochemical oxygen demand (B0D) and fourth was daily flow interval.

Multiple permit violations in the beginning led to categorization as a significant non-compliant violator.

Findings and actions

As it turned out, the solution was fairly straightforward!

The first challenge was to maintain consistent compliant discharge of Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS).

Analysis showed while using the correct wastewater treatment procedure, clarified water was discharge permit compliant. It also showed that based on a continuous flow rate and resulting available contact time in each of the five in-series batch tanks, solids separation was incomplete due to limited residence time in each in-series batch tank.

The solution to this treatment condition was to only treat the wastewater chemically in the batch tanks without attempting solids separation and process all of the chemically treated waste through a plate-and-frame press for solids separation. 

The filtrate from the plate-and-frame press satisfied the discharge permit requirements for BOD and TSS.

The pH of the clarified wastewater, filtrate effluent from the press, was at a low 3.0. Minimum pH for compliant discharge was a pH => 6.0. So the simple solution to resolve pH discharge permit requirements was to use a dedicated tank for pH adjustment.

This tank was approximately 2,000 gallons volume capacity and we maintained a minimum tank level of at least 60 percent, or 1,200 gallons, which was always pH-adjusted in the tank in the tank at around 7.5 pH.

We pumped approximately 50 gallons per minute (GPM) of low pH press filtrate into the dedicated pH adjustment tank for about 25 minutes contact time. The discharge of the same 50 GPM flow rate was easily maintained consistently at approximately pH of 7.5, which was a compliant discharge. So that was that.

Foam acted as an interference in the flow measurement at the flume for discharge. A foam control procedure resolved that part of the problem.

The point is, it can be done!

Known in the industry as “Wastewater Dan,” Daniel Theobald, proprietor of Environmental Services, is a professional wastewater and safety consultant with 25 years of experience.

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