TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The hydrogenotrophic methanogens belong to the order Methanomicrobiales or to the order Methanobacteriales. The hydrogenotrophic methanogens comprise Methanospirillum hungatei, or a related hydrogenotrophic methanogen, where the Methanospirillum hungatei, or a related hydrogenotrophic methanogen represent at least about 95% of hydrogenotrophic methanogens belonging to the order Methanomicrobiales in the culture. The hydrogenotrophic methanogens comprise Methanolinea tarda or a related hydrogenotrophic methanogen. The culture comprises wastewater sludge.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method, prior to being added to the system, the culture was grown in the presence of oxygen or an organic acid or its salt. The culture is capable of metabolizing propionate. The culture is added to the system at a rate of at least about 10 mg volatile suspended solids (VSS)/L-day. In the method, prior to adding the culture, partial pressure of hydrogen in the system is at least 10-6 atm; the culture is added to the system for a period of time that is sufficient to reduce the partial pressure of hydrogen in the system to less than 10-6 atm; and prior to adding the culture, the system has a pH of not greater than 6.6. The culture is added to the system for a period of time that is sufficient to raise the pH above 6.6. In the method, prior to adding the culture, the system has a propionic acid concentration of at least 500 mg/l. The culture is added to the system for a period of time that is sufficient to reduce propionic acid concentration below 500 mg/l. In the method, prior to adding the culture, the system has a soluble chemical oxygen demand (SCOD) of at least 1000 mg/l. The culture is added to the system for a period of time that is sufficient to reduce the SCOD to less than 1000 mg/l. Preferred Components: The organic acid is a straight chain or branched carboxylic acid having at least three carbon atoms. EXAMPLE - No suitable example is given.