Abstract
M.Sc. (Chemistry)
Volatile fatty acids (VFAs) play a pivotal in the process of nutrient removal by biological processes particularly the enhanced biological nutrient removal process with a side-stream elutriation process using activated sludge. These acids are said to act as intermediates which provide feed for the organisms in a biological phosphorus and nitrogen removal (BNPR) system, such as phosphorus-accumulating organisms (PAOs) and nitrate-accumulating bacteria (NABs).
In wastewater treatment plants, VFAs play a vital role as intermediate organic compounds during the fermentation processes which generate methane gas and when present at elevated levels they are known to cause microbial stress, acidification as well as the poor performance of anaerobic digesters. For these reasons, the routine monitoring of VFA levels in wastewater treatment plants is crucial as they will act as indicators of the efficiency and optimal operation performance of the anaerobic digesters. Normally the VFAs that are commonly produced during the anaerobic fermentation process include acetic acid, propionic acid, butyric acid, and valeric acid and of these, acetic and propionic acids form the major VFAs that are generated, thus the yields of these two compounds provide a useful measure of the anaerobic digester performance. For example, the ratio of propionic acid to acetic acid is always used as an indicator of digester imbalance while high concentrations of acetic acid (e.g. > 800 mg/ℓ) or a propionic acid to acetic acid ratio greater than 1.4 is an indication of digester failure.
This study was thus aimed at establishing the complete VFA profile at the Johannesburg Water (JW) Northern Works Wastewater Treatment Plant in Johannesburg, South Africa, by developing analytical methods to quantify the VFAs in the wastewater treatment plant. In addition, the level of VFAs quantified was used to evaluate the efficiency of the fermentation treatment process in wastewater treatment systems in order to give an indication of the bacterial activities in the systems; to determine the ratios of the VFAs, especially the propionic acid to acetic acid ratio, to establish the performance and efficiency of various wastewater treatment plants; and to determine whether there is any imbalance in the anaerobic digesters.
Two analytical methods for the determination of VFAs were successfully developed and applied to real wastewater samples. The first method developed was a liquid-liquid extraction method using gas chromatography time-of-flight mass spectrometry (GC-TOF-MS. The method involved two extracting solvents, namely dichloromethane (DCM) and methyl-tert-butyl ether (MTBE). The extraction capacity of these two solvents was compared. The second method successfully developed and optimised headspace-solid phase microextraction (HS-SPME) using GC-TOF-MS. The microextraction fibre used was a polydimethylsiloxane (PDMS) fibre. Studies to validate the developed methods were also carried out by calculating the limit of detection (LOD) and limit of quantification (LOQ). Ratios of propionic acid to acetic acids were determined as well as the concentrations of acetic acid for both developed methods in order to evaluate the performance and efficiency of the treatment process.
The results of the study show that the extraction procedure using solvents DCM and MTBE showed that exhaustive extraction was achieved by MTBE. Higher concentrations of acetic acid and propionic acid were obtained by MTBE. The overall fermentation performance for all three units in the period when the samples were collected, which was measured by the ratio of propionic to acetic acid was good since the ratio did not exceed 1.4. The acetic acid concentration in mg/ℓ was < 800 mg/ℓ for all units thus it can be said that the reactor balance was maintained for the period studied. The method LOD ranged from 0.034 mg/ℓ to 0.21 mg/ℓ and the method LOQ ranged from 0.11 mg/ℓ to 0.70 mg/ℓ.
Optimisation of extraction parameters was achieved for headspace solid-phase microextraction using the PDMS fibre method. The extraction method was conducted for 60 min using a sample volume of 4 mℓ and the amount of sodium salt added was 1.50 g. The desorption temperature and time was 210ºC and 5 min, respectively. The LOD values ranged from 0.079 to 2.07. The PDMS fibre was found to be suitable for extracting higher carbon chain fatty acids as compared to lower carbon chain fatty acids. The plant performance for the period studied was on par as indicated by ratios of propionic acid to acetic acid which all fell below 1.4. There were no digester failures for the period studied. The acetic acid concentration in mg/ℓ was < 800 mg/ℓ for all 3 units.