Search

Effect of hydraulic retention time (HRT) on the acidogenesis of dairy wastewater was studied using an upflow reactor at pH 5.5, 37 degrees C, and six HRTs ranging from 4 to 24 h. Results showed that the degree of acidification increased rapidly with HRT from 28.2% at 4 h to 54.1% at 12 h; further increase of HRT to 16 and 24 h only increased acidification slightly to 55.8 and 59.1%, respectively. The biodegradability of the three major constituents in dairy wastewater increased with HRT, following the order of carbohydrates > proteins > lipids. The predominant products were acetate, propionate, butyrate, lactate, and ethanol.

The formation and characteristics of intracellular polymers in aerobic sludge of sequencing batch reactors were investigated at ambient temperature under balanced nutrient conditions. Three substrates of different chemical natures, including fatty acid (acetate), carbohydrate (glucose), and aromatic (benzoate), were fed to individual reactors. When substrates were initially in excess, the sludge in all reactors was capable of converting soluble substrates into intracellular polymers under aerobic conditions. Acetate (up to 27%) and benzoate (up to 51%) were converted to poly-13-hydroxybutyrate, whereas glucose (up to 33%) was converted to intracellular carbohydrates. The initial substrate depletion rates were 208-243 mg-C/g-VSS/h for acetate, 491-590 mg-C/g-VSS/h for benzoate, and 405-558 mg-C/g-VSS/h for glucose. When external substrates were absent in the mixed liquor, the intracellular polymers could be consumed by the sludge for endogenous respiration under aerobic conditions or as a carbon source for denitrification under anoxic conditions. These results suggest a dynamic metabolic mechanism in the sequencing batch reactors.

The formate-utilizing sludge was first enriched in a chemostat reactor for over 90 days; at steady state, the sludge yield averaged 0.066 mg volatile suspended solids (VSS)/mg chemical oxygen demand (COD). The methanogenic characteristics of this sludge were then investigated in three series of batch experiments at 37 [degrees] C using formate, acetate, and [H.sub.2]/C[O.sub.2], individually, as substrates. At pH 6.4-8.0, the formate-degrading rate averaged 0.76 mg/mg VSS/h (6.35 mg COD/mg VSS/d). At pH 3.0, the sludge completely lost its bioactivity, and required a lengthy recovery period to regain a fraction of its bioactivity after the pH was adjusted to pH 7.1. The sludge was also able to utilize [H.sub.2]/C[O.sub.2] as substrate at an average rate of 0.0167 mg [H.sub.2]/mg VSS/h (3.21 mg COD/mg VSS/d). At pH [less than or equal to] 8.0, the sludge degraded acetate at a very low rate of 3.0 [[micro]gram]/mg VSS/h (0.077 mg COD/mg VSS/d). The sludge exhibited a slight homoacetogenic activity at pH > 8.0 using formate as substrate; the homoacetogenic reaction using [H.sub.2]/C[O.sub.2] as substrate was thermodynamically infeasible, according to chemical free energy analysis.

Experiments were conducted at 37 [degrees] C in an upflow anaerobic sludge blanket reactor treating waste-water containing phenol (200 mg/L), m-cresol (100 mg/L), and nitrate at various concentrations. Results show that anaerobic sludge was able to conduct denitrification without much acclimation. Denitrifiers outcompeted methanogens for substrates for carbon and electron supplies. They were able to use phenol and m-cresol as substrate without a carbohydrate cosubstrate. Denitrifying 1 g of [Mathematical Expression Omitted] required 3.34 g of chemical oxygen demand. Methanogenesis occurred only at chemical oxygen demand/[Mathematical Expression Omitted] ratios greater than 3.34. At the ratio of 5.23, over 98% of phenol but only 60% of m-cresol were degraded jointly by denitrifiers and methanogens with I day of hydraulic retention. At ratios less than 3.34, methanogenesis ceased to take place and denitrification became incomplete because of insufficient supply of substrate. Batch tests further confirmed that degradation of m-cresol was enhanced not only by the presence of nitrate, but also by the presence of either sucrose or phenol as cosubstrate.

Wastewaters containing benzoate and sulfate were treated in two upflow anaerobic sludge blanket (UASB) reactors at 34-37 [degrees] C for 320 d. The sulfate concentration was increased stepwise in Reactor-A up to 7,500 mg/L, and was kept mostly constant at 3,000 mg/L in Reactor-B. Both reactors removed over 98% of organic chemical-oxygen demand (COD) for sulfate up to 6,000 mg/L, despite the fact that the mixed liquor contained up to 769 mg S/L of total sulfides and up to 234 mg S/L of dissolved [H.sub.2]S. Sulfate-reducing efficiency decreased with the increase in sulfate concentration, but increased with time at each sulfate concentration. Reactor-B consistently reduced 89% of sulfate. However, both organic COD removal and sulfate-reducing efficiencies of Reactor-A dropped drastically at 7,500 mg [Mathematical Expression Omitted], and showed no sign of recovery after 50 d. The system failure was likely due to the increased sulfate, instead of sulfide, toxicity. From the COD balance, 93.4% of COD removed was converted to methane instead of sulfides, with a net sludge yield of 0.047 g volatile suspended solids (VSS)/g COD. The sulfur balance was over 97%.

Wastewater containing high concentrations of cornstarch was treated in two 8.5-L upflow reactors: upflow anaerobic sludge blanket (UASB) and a modified anaerobic filter (AF). Both reactors were operated in parallel at 37 [degrees] C and pH 6.8-7.9 over 510 d with increased loadings of chemical oxygen demand (COD). Results showed that, in spite of having different sludge concentration profiles, both reactors, with 12 h of hydraulic retention time, removed an average of 95.3% of COD from wastewater containing up to 45,000 mg-COD[center dot][L.sup.-1] (loading rates up to 90 g-COD[center dot][L.sup.-1][center dot][d.sup.-1]). At loading rates up to 75 g-COD[center dot][L.sup.-1][center dot][d.sup.-1], only 0.42% and 0.80% of starch particulates remained unhydrolyzed and were washed out by the effluent from UASB and modified AF reactors, respectively. Of the COD removed, 85.9% was converted to methane and the rest was converted to biomass with a net sludge yield of 0.10 g-VSS[center dot]g-[COD.sup.-1]. Both reactors failed at loading rates higher than 90 g-COD[center dot][L.sup.-1][center dot][d.sup.-1], because of severe washout of solids. The sludge granules had a methanogenic activity of 1.90-1.92 g-methane-COD[center dot]g-[VSS.sup.-1][center dot][d.sup.-1] and consisted of various trophic bacteria in the order of [10.sup.8]-[10.sup.9] m/L.