Updated: Nov 21, 2020
A lot of my work is with high-strength industrial wastewater. Issues that arise with any given industrial waste stream may consist of, but are not limited to, the following:
1. High chemical oxygen demand (COD) that exceeds the treatment capacity of the onsite wastewater treatment system (activated sludge or biological treatment unit).
2. In the case where an industrial plant discharges directly to the local publicly owned treatment works (POTW) or municipal wastewater treatment plant, a high-strength COD results in high surcharges being charged back to the industrial facility.
3. Complex COD, also described as refractory or recalcitrant COD, that cannot be sufficiently oxidized in the onsite bioreactor, leading to high COD concentrations in the effluent.
4. Inhibitory compounds (e.g., benzene, phenol) that interfere with, or hinder, the performance of, the activated sludge system in the oxidation (degradation) of the incoming organic load, again leading to high COD concentrations in the effluent. This can lead to violations of the NPDES (National Pollutant Discharge Elimination System) permit.
In this post I want to discuss phenol in the wastewater, a chemical compound perceived by many to be hard-to-treat and highly inhibitory to microorganisms. Frequently, I deal with this chemical in landfill leachate and refinery wastewater.
According to Berne and Cordonnier, in refineries, phenols can be present in massive amounts in spent caustic and, to a lesser extent, in delayed coking effluents (1995). The biodegradability of phenols was still considered debatable in the 1960s. In actual fact, it is good and well controlled in aerobic biological purification as long as the compounds are fed in at a relatively constant rate and there are no coexisting inhibiting agents such as S2− in overly strong concentrations. In coking plants, ammonia process water containing up to 2.5 g•L−1 of phenols can be treated directly in this way. The removal efficiency is over 99.5%. The biodegradation process is thought to be as follows:
Leachate Example - COD
Let's look at a leachate sample where the issues were a high COD concentration combined with a high phenol concentration. A variety of chemical oxidation programs were tested to determine which would provide the greatest (and most economical) reduction in both parameters. The untreated or control sample had a COD concentration of 63,000 mg/L and a phenol concentration of 76 mg/L.
Permanganate, sodium or potassium, is effective in reducing phenol. Fenton's Reagent (metal catalyst + hydrogen peroxide) is effective in reducing COD. Both programs were tested and the results are shown graphically below.
As shown in the horizontal bar chart above, the three treatments, from the top down, are:
1. pH adjusted from near neutral to 4; iron concentration of 400 ppm; hydrogen peroxide concentration of 4,000 ppm
2. pH adjusted from near neutral to 11; sodium permanganate concentration of 2,000 ppm
3. pH adjusted from near neutral to 9; sodium permanganate concentration of 2,000 ppm
The percent reduction in COD is shown in the graph below.
The reduction in COD is disappointing! Though additional testing included varying the pH, dosages for these two chemical programs, and duration of chemical reaction time, a satisfactory reduction in the COD of this leachate was never achieved. This is a frustrating result but it happens and it's always hard to know why. With leachates in particular, depending on the age and natural of materials that have gone into the landfill, it is difficult to know what combination of chemical compounds is interfering with the chemical oxidation process.
Leachate Example - Phenol
Though COD reduction efforts failed, success was achieved with phenol reduction. The reduction in the phenol concentration is shown below.
Increasing the pH of the leachate from near neutral to 11 and then adding 2,000 ppm of sodium permanganate resulted in a phenol concentration of 1.4 mg/L providing a 98.2% reduction.
For a given sample, where the specific goal is phenol reduction, how long does the chemical reaction take to oxidize the phenol? Well, that depends. Phenol oxidation can be achieved in 20-30 minutes but samples that contain complex organics can require several hours of reaction time. That means phenol reduction is going to be a batch process, requiring tankage for storage of the waste stream to allow the reaction to go to completion.