Allen C. Chao

Influence of process loading intensity on sludge clarification and thickening characteristics

Abstract The separation of microbial mass in an activated sludge system depends on the formation of large settleable sludge flocs. Microbial cells agglomerate into small clumps and subsequently into sludge flocs by a natural bioflocculation process. The resulting biological mass may exist as individual cells, small aggregates, or large flocs in the aeration basin effluent. In the secondary clarifier, only large flocs are separated by sedimentation while individual cells and small aggregates are washed out as effluent suspended solids. In the present study, the separation of sludge by sedimentation was shown to be closely related to the surface roughness of the resulting flocs. Normal sludges were observed to have relatively smooth surfaces while filamentous and zoogloeal bulking sludges were noted to have rough and diffused surfaces. The filamentous growths and the non-filamentous protuberances on the bulking sludge surface serve to increase the frictional drag between the moving solid and the liquid. This contributes to a lower settling velocity and associated dewatering rate of bulking sludges. Conversely, the quantity of non-settleable solids was shown to be related to the quantity of extracellular biopolymers excreted by the microorganisms. The quantity of biopolymers produced decreased for higher Process Loading Intensity or lower sludge age levels thereby resulting in poor aggregation of cells into clumps and subsequently into flocs. This contributes to low clarification efficiencies of the activated sludge process.

BMP test on chemically pretreated sludge

Municipal waste activated sludge (WAS) was treated with NaOH to solubilize the particulate organic matter in order to improve its digestibility when the sludge was stabilized in an anaerobic digestion process. For the WAS of 1% TS treated with 20 and 40 meq/l NaOH at ambient temperature for 24 h, the SCOD/TCOD values increased from 3.5 to 39 and 55%, respectively. Results of the BMP (biochemical methane potential) test showed that recoveries of carbon and nitrogen reached 94–105% and the methane produced was 349 ml (at 1 atm and 35°C) for 1 g of COD removed. The improvement in VS removal for sludge treated with 40 meq/l of NaOH was as high as 41% over the control sludge sample, with COD removal and gas production improved by 30 and 34%, respectively, over the control. Results of the cumulative COD removal appeared to follow first-order reaction kinetics. Parameters of the first-order model such as ultimate biodegradable COD (Lu), reaction rate coefficient (k) and ultimate biodegradability (μ), were calculated using the Thomas method. The BMP test can be used as a valuable tool to study the kinetics and efficiency of anaerobic digestion processes.

Modified nernst model for on-line control of the chemical oxidation decoloring process

A modified Nernst equation was used to describe the decoloring reaction using the Oxidation-reduction potential (ORP) as an on-line monitoring and control parameter of the chemical reaction. In the modified equation, an “S” term is defined to indicate the oxidative potential (or decoloring potential) of dyes being studied. The laboratory study utilizes sodium hypochlorite (NaOCl) to reduce the color of wastewater spiked with textile dyes. Five dyes were used in preparing the wastewater samples: methyl red (MR), methyl violet (MV), methyl blue (MB), malachite green (MG) and methyl orange (MO). The color was measured by use of an automatic ADMI (American Dye Manufacturer Institute) measurement system in which a visible spectrophotometer was connected to a personal computer and the samples were scanned from 400 nm to 700 nm with a 10 nm interval each step. During the batch decoloring studies, several factors including ADMI, ORP, temperature and pH were continuously monitored with the computer. In this study, the potential of the decoloring compound of the dyes studies are in the following order: MG>MV>MO>MR>MB, with numeric ratios of 2.08 : 1.78 : 1.78 : 1.28 : 1.00, respectively.

The pretreatment of acrylonitrile and styrene with the ozonation process

Acrylonitrile and styrene are used as the raw materials for manufacturing acrylic fiber, thus they are often found as pollutants in the petrochemical wastewater. This study utilizes ozone to decompose the organic nitrogen contained in acrylonitrile and styrene, and the oxidation process was monitored using on-line measurements of oxidation-reduction potential (ORP) and pH. The efficiency of organic nitrogen decomposition was also estimated based on the COD, organic nitrogen, TOC, ammonia-N, nitrite, and nitrate measurements. Both the initial pH and alkalinity are observed to affect the degradation rate of organic nitrogen. The acrylonitrile sample with the lowest initial pH value (i.e., 4.0) has a shorter t1/2 of 18.9 min and that for samples of the highest initial pH (i.e., 11) was 34 min. The alkalinity of one acrylonitrile sample was boosted by adding 500 mg/l CaCO3, to simulate the field ABS (Acrylonltrde-Butadiene-Styrene) wastewater effluent. It was observed that within a short ozone contact time, the acrylonitrile sample spiked with 500 mg/l CaCO3 had the highest COD decomposition rate of 0.411 mm−1, or 1.3 times more than that for samples without addition of CaCO3. Results of the ozonation process can be fitted with a modified Nernst equation for the various pH conditions. Additionally, the ozone treated synthetic ABS sample shows a faster COD removal rate in the subsequent biological process than those samples without ozone treatment.

Destabilization of biological solids with ferric chloride

Abstract When ferric chloride is applied to a mixture of sludge pinpoint flocs and dispersed cells, a preferential removal of the flocs is observed at low dose levels. The dispersed cells, which constitute a small percentage of the total suspended solids, require higher dose of coagulant for their complete removal. On a weight basis, the presence of dispersed cells in an activated sludge effluent will cause a significant increase in the stoichiometric quantity of ferric chloride required for complete removal of all suspended biological solids. The coagulation process can be enhanced using a two-step pH control method. Instead of maintaining the solution pH constant at the optimum level during the period of addition of ferric chloride, it was allowed to drop and then adjusted back to the optimum level. The two-step pH control method, which can be implemented by delaying the addition of bases required for pH adjustment, results in more efficient use of the coagulant for destabilizing the biological solids. Results of laboratory studies employing this technique have shown a 25% reduction in the ferric chloride dose requirements.

Decoloring Mechanisms of Dye Stuff by Chemical Oxidation

This study utilizes sodium hypochlorite (NaOC1) to reduce the color of synthetic dye solutions. Five dyes including methyl red (MR), methyl violet (MV), methyl blue (MB), malachite green (MG) and methyl orange (MO) were studied. The color was measured using an automatic ADMI (American Dye Manufacturer Institute) measurement system. A visible spectrophotometer is connected to a personal computer so that samples can be scanned and the results are recorded automatically. During the batch decoloring study, several factors: ADMI, ORP, temperature and pH were also monitored continuously using the computer and makes it possible to have an automated decoloring process.