When sludge is allowed to settle in a digester, a supernatant develops. Anaerobic digester supernatant is commonly returned to the head of wastewater treatment plants and mixed with the influent (Figure 21.1). Although the supernatant is relatively small in volume, it contains dissolved and suspended organic and inorganic materials. These materials add suspended solids, nutrients (nitrogen and phosphorus), and organic compounds to the influent.
The returned materials may cause a variety of operational problems (Table 21.1). Therefore, wastewater treatment plants that are not achieving significant liquidsolid separation in anaerobic digesters should consider discontinuing the practice of returning supernatant to the headworks of the plant.
Increased chlorine demand may occur because of the presence of excess ammonium ions in the supernatant. Volatile fatty acids, volatile organic compounds, volatile sulfur compounds, and hydrogen sulfide released from the supernatant in the turbulent headworks of a wastewater treatment plant may contribute to malodor problems.
Sludge bulking in the activated sludge process may occur through the weakening of floc particles by excess total dissolved solids (TDS) or the rapid and unde-sired growth of filamentous organisms. The presence of sulfides in the supernatant may trigger the growth of sulfide-loving filamentous organisms such as Beggiatoa spp. and Thiothrix spp., whereas the presence of readily degradable organic compounds may trigger the growth of foam-producing filamentous organisms such as Microthrix parvicella and Nocardioforms.
TABLE 21.1 Operational Problems Associated with the Return of Digester Supernatant to the Head of a Wastewater Treatment Plant
Increased chlorine demand Malodor problems Sludge bulking
Undesired impact of high concentrations of nitrogen and phosphorus
Several operational problems are associated with the presence of high concentrations of nitrogen and phosphorus. These problems include possible permit violations, nitrification, denitrification, and excess growth of algae in secondary clarifiers.
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