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 other plants.
The best way to determine relevancy is to look at an example project’s details, including its scope, data & information gathered, findings and actions taken.
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.
The continuous nature of the violations made for a potentially serious situation. 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.
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 $3,000.
The project was meant to identify the source of the violations, identify corrective actions and eliminate violations.
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.
That concluded this project.