Investigations of the impact of solid residence time (SRT) on microbial ecology and performance of enhanced biological phosphorus removal (EBPR) process in full‐scale systems have been scarce due to the challenges in isolating and examining the SRT from other complex plant‐specific factors. This study performed a comprehensive evaluation of the influence of SRT on polyphosphate‐accumulating organisms (PAOs) and glycogen‐accumulating organisms (GAOs) dynamics and on P removal performance at Clark County Water Reclamation District Facility in Las Vegas, USA. Five parallel treatment trains with separated clarifiers were operated with five different SRTs ranging from 6 to 40 days. Microbial community analysis using multiple molecular and Raman techniques suggested that the relative abundances and diversity of PAOs and GAOs in EBPR systems are highly affected by the SRT. The resultant EBPR system stability and performance can be potentially controlled and optimized by manipulating the system SRT, and shorter SRT (<10 days) seems to be preferred.
Practitioner points
Phosphorus removal performance and kinetics are highly affected by the operational solid residence time (SRT), with lower and more stable effluent P level achieved at SRT < 10 days.
Excessive long SRTs above that needed for nitrification may harm EBPR performance; additionally, excessive long SRT may favor GAOs to dominate over PAOs and thus further reducing efficient use of rbCOD for EBPR.
Microbial population abundance and diversity, especially those functionally relevant PAOs and GAOs, can impact the P removal performances, and they are highly dependent on the operational solid residence time.
EBPR performance can be potentially controlled and optimized by manipulating the system SRT, and shorter SRT (≤10 days) seems to be preferred at the influent rbCOD/P ratio and environmental conditions as in the plant studied.
Lake Mead was formed by the construction of Hoover Dam. It is the largest man made reservoir in the United States. Lake Mead started to fill in 1936 and finished in 1939. For the next 25 years the only water quality concerns were solids settling into the bottom of the reservoir. In 1956 both the City of Las Vegas (197.2 L/s) (4.5mgd) and Clark County (52.6 L/s) (1.2mgd) began discharging wastewater into the Las Vegas Wash, which flows into Lake Mead. At that time the Total Phosphorus (Total P) concentrations in the effluent typically ranged between 5 to 7 mg/L. Because of the large quantities of nitrogen fixing blue-green algae in the Las Vegas Bay, it was believed to be Nitrogen limited until Total P removal was begun in 1980. In 1965 the Bureau of Reclamation (BOR) began filling Lake Powell. As a result the level of Lake Mead dropped dramatically. It was at this time that the algae blooms in Lake Mead began. By the spring of 1967 the newspapers started carrying articles about the algae blooms. In 1972, Southern Nevada Water Authority began using a drinking water intake located downstream of the wastewater discharges. This began the use of Indirect Potable Reuse (IPR) in the Las Vegas
Aerial Photo of Hoover Dam and Las Vegas Bay in Lake MeadValley. Today 100% of the 7013 L/s (160mgd) of wastewater flows from the Las Vegas Valley are reclaimed in this way.
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