Coastal Zone Act Reauthorization Amendments (CZARA, Section 6217) necessitate the requirement that onsite wastewater disposal units located near impaired surface waters or groundwater to provide at least 50% nitrogen removal. Approximately 38% of Hawaii households use onsite systems including septic tanks and cesspools that cannot meet this requirement. Upgrades to aerobic treatment units (ATUs) are a possible compliance solution. In Hawaii, ATUs must meet National Sanitation Foundation Standard 40 (NSF40) Class I effluent criteria. Previously, a multi-chamber, flow-through, combined attached/suspended growth type ATU (OESIS-750) and presently, a sequencing batch type ATU (CBT 0.8KF-210) were evaluated for NSF40 compliance, nutrient removal capability (NSF245), and adaptability for water reuse (NSF350). Both units easily achieved the NSF40 Class I effluent criteria. While the OESIS-750 achieved only 19% nitrogen removal, the CBT unit achieved 81% nitrogen removal, meeting the NSF245 criteria and CZARA requirements for applications in critical wastewater disposal areas. In addition, the CBT consistently produced effluent with turbidity less than 2 NTU (NSF350) and UVT254 greater than 70%, facilitating the production of unrestricted-use recycled water.

INTRODUCTION

Approximately 25% of households in the USA use onsite disposal systems (OSDSs) including aerobic treatment units (ATUs), septic tanks and cesspools to manage their wastewater (USEPA 2014); OSDSs serve approximately 38% of residents of the State of Hawaii (DOH 2014). Because cesspools and septic tanks provide only partial treatment, they are often considered a temporary solution that will eventually be replaced by a public centralized sewer system (USEPA 2002). However, adequately managed and maintained decentralized wastewater systems (especially those consisting of mostly ATUs) are a cost-effective and long-term option for meeting public health and water quality goals, particularly in low density population areas (USEPA 1997; Seabloom et al. 2005; Lamichhane 2007). Inappropriate siting or design or inadequate long-term maintenance and management are considered the main reasons for failure of these systems (Swartz et al. 2006; Babcock et al. 2014). Poorly maintained OSDSs pollute (shallow) ground water, which can quickly discharge to and pollute surface waters (Szabo et al. 1996; Modica et al. 1998). In Hawaii, nonpoint source (NPS) pollution has been recognized as the largest source of water quality problems for lakes, streams, wetlands and coastal waters (SOH 2013).

In order to protect coastal waters from nonpoint pollution sources, the United States Environmental Protection Agency (EPA) introduced the Coastal Zone Act Reauthorization Amendments (CZARA) of 1990. CZARA (Section 6217) includes the Coastal Nonpoint Source Pollution Control Program, which requires that states implement measures to prevent NPS pollution in coastal waters by following CZARA guidelines. The CZARA requirements for treatment units that are liable to adversely affect nitrogen-limited surface waters is that they reduce nitrogen (N) loadings by at least 50% to ground water that is closely connected to surface waters. Conventional ATUs are relatively efficient in removing biochemical oxygen demand (BOD) and total suspended solids (TSS) but most of the N is released as nitrate rather than transformed to nitrogen gas via denitrification. Denitrification adds complexity to the system (Oakley 2005) but is necessary in order to remove nitrogen prior to disposal.

Onsite technologies like ATUs could be sustainable alternatives to conventional wastewater management systems if they can meet discharge requirements in Critical Wastewater Disposal Areas (CWDAs) under CZARA regulations (50% N removal). The State of Hawaii requires that ATUs meet National Sanitation Foundation Standard 40 (NSF40) Class I requirements (effluent BOD5 and TSS less than 30 mg/L on 30-d average and at least 85% removal, pH between 6 and 9) to be utilized in Hawaii. However, ATUs without proper maintenance will not perform any better than septic tanks and cesspools (Roeder & Brookman 2006; Babcock et al. 2014). The NSF245 standard additionally requires 50% nitrogen removal (NSF 2014). The Hawaii requirements for producing unrestricted use recycled water from secondary treated wastewater indicate that post-filtration turbidity must be less than 2 NTU and UV transmittance at 254 nanometers (UVT254) must be at least 55% for media filtered effluent (DOH 2002).

The CBT 0.8KF-210 unit (CBT), a sequencing batch reactor ATU, was evaluated to determine whether it could meet the requirements of NSF40, NSF245, NSF350 and CZARA nitrogen requirements for CWDA. The NSF40 and 245 protocols require a 6-month continuous testing period of specific flow patterns followed by a series of specific (stress) tests (NSF 2014). Previously, a different ATU employing a combined attached and suspended growth process (OESIS-750) was similarly evaluated relative to NSF40 Class I requirements and to check effluent suitability for recycling (Babcock et al. 2004; WRRC & ES 2008). This research focuses on CBT evaluation and compares its performance to the previously tested unit.

MATERIALS AND METHODS

The CBT unit was sited at the Honouliuli Wastewater Treatment Plant (HWWTP) on the island of Oahu, Hawaii. The 1,000 gallon (1 gallon ≈ 3.8 L) unit consisted of a 10.5 feet (∼3.2 m length) by 6 feet (∼1.83 m diameter) chamber that utilizes a sequencing batch activated sludge reactor process to treat 800 gallons per day of wastewater, giving a hydraulic retention time of 1.25 days. Figure 1 shows a schematic of the CBT installation.

Figure 1

Schematic of CBT 0.8KF-210 ATU assembly used for the study.

Figure 1

Schematic of CBT 0.8KF-210 ATU assembly used for the study.

The water level within the unit is designed to fluctuate between 2.9 and 4.0 feet (for flow equalization) with an average water depth of 3.5 feet. A stilling basin separates wastewater entering the chamber from the main treatment area. The wastewater experiences adjustable-duration cycles of aeration in the main chamber. Blowers operate on a timer and turn on and off every 2 hours, exposing the mixed liquor (ML) to aerobic (mixing and nitrification, 2 hours) and anoxic (settling and denitrification, 2 hours) conditions. Before blowers turn back on, a floating decanter, a submersible pump and float switches are used to remove effluent from the unit, but only when the unit experiences water levels greater than 2.9 feet (decant is 1 hour at the end of the aeration off cycle).

Two programmable samplers (ISCO Model 3700, Lincoln, NE, USA) were utilized to collect influent and effluent samples 5 days per week, each hour during those times when the unit received influent (NSF40 protocol). Influent and effluent samples were composited in proportion to the influent flow pattern and were analyzed for BOD5 and TSS. Grab samples from the aeration tank (ML) were also collected 5 days per week and in-situ measurements of dissolved oxygen (DO), temperature, and pH were also made. All analytical measurements were performed according to procedures detailed in the 19th edition of Standard Methods for the Examination of Water and Wastewater (APHA 1995). Analytical methods 5210 B and 2540 D were used for BOD5 and TSS measurements, respectively. The ML DO was measured using method 4500-G, temperature by thermistor on DO probe, pH by method 4500-H, and settleable solids by method 2540-F. Effluent grab samples were also monitored periodically for turbidity, ultraviolet transmittance at 254 nm (UVT254), and oil and grease using standard methods 2130 B, 5190 B, 9222 D, and 5520 B, respectively.

Additional samples were collected from both chambers of the treatment unit at various intervals and measurements were made for organic N, orthophosphate, and total phosphorus (TP) using methods 4500-NO3 E, 4500-P C and 4500-P C, respectively. Effluent grab samples were also taken three times during the evaluation period (on day 29, 60, and 119) and evaluated for color, threshold odor, oily film, and foam by diluting it to 1:1,000 with distilled water. There are no requirements in NSF40 for temperature, DO and settleable solids concentrations but it does specify that they must be measured. Influent and effluent Ammonia-N, total nitrogen (TN), TP, turbidity, and UVT254 were also measured three times per week to monitor nitrogen removal (NSF245) and to check on suitability for recycling/reuse (NSF350).

During the 6-month standard performance test, the system was fed raw wastewater according to the NSF40 protocol (see Supplementary Information (SI), page i, available online at http://www.iwaponline.com/wst/071/172.pdf). After the completion of the standard performance test (days 0–202), the system was subjected to a series of four stress tests, which lasted approximately 2 months, during which the system was given 1 week of recovery (normal daily flow pattern) between stress tests. Stress tests I (wash day), II (working parent), III (power failure), and IV (return from vacation) were conducted on days 284–288, 298–302, 307–310, 316–325, respectively. The system performance was evaluated to determine compliance with NSF40 Class I effluent requirements as well as CZARA requirements. NSF40 test protocols for system loading during standard performance and stress tests periods, and Class I effluent requirements can be found on the NSF website (NSF 2014).

RESULTS AND DISCUSSION

Standard performance period

The CBT was brought online for continuous operation and the 6-month standard performance period was completed on day 202. The average temperature and pH within the unit were 27.7 ± 1.1 °C and 7.7 ± 0.23 °C, respectively. The influent BOD5 and TSS were similar to typical medium-strength municipal wastewater (198.3 ± 77.2 mg/L) (see SI, Figure S1, available online at http://www.iwaponline.com/wst/071/172.pdf). Despite large variations in influent BOD5, the effluent BOD5 remained fairly stable (4.6 ± 5.5 mg/L) (Figure 2) and the average removal was nearly 98% (Table 1). The unit was tested under conditions that were similar to the OESIS-750 unit, which produced an average effluent BOD5 of 13.9 ± 6.0 mg/L and achieved a BOD5 removal rate of 90% (Babcock et al. 2004).

Table 1

Performance comparison of CBT 0.8KF-210 and OESIS-750 units during a 6-month standard performance period

  CBT 0.8KF-210 OESIS-750 
Parameters Average value Standard deviation Average value Standard deviation 
BOD 
 Influent BOD5 (mg/L) 198.3 77.2 146.4 20.3 
 Effluent BOD5 (mg/L) 4.6 5.5 13.9 6.0 
 BOD removal (%) 97.7  91.0  
Solids 
 Influent TSS (mg/L) 241.5 67.5 128.0 27.6 
 Effluent TSS (mg/L) 2.7 2.8 13.1 6.9 
 TSS removal (%) 98.9  89.7  
Aeration tank (ML) 
 DO (mg/L) 2.3 1.0 3.4 1.3 
 pH 7.1 0.23 7.4 0.2 
 Temperature (°C) 27.7 1.1 25.4  
  CBT 0.8KF-210 OESIS-750 
Parameters Average value Standard deviation Average value Standard deviation 
BOD 
 Influent BOD5 (mg/L) 198.3 77.2 146.4 20.3 
 Effluent BOD5 (mg/L) 4.6 5.5 13.9 6.0 
 BOD removal (%) 97.7  91.0  
Solids 
 Influent TSS (mg/L) 241.5 67.5 128.0 27.6 
 Effluent TSS (mg/L) 2.7 2.8 13.1 6.9 
 TSS removal (%) 98.9  89.7  
Aeration tank (ML) 
 DO (mg/L) 2.3 1.0 3.4 1.3 
 pH 7.1 0.23 7.4 0.2 
 Temperature (°C) 27.7 1.1 25.4  
Figure 2

Effluent BOD5 (mean concentration 4.6 mg/L) and TSS (mean concentration 2.8 mg/L) during 6-month standard performance period.

Figure 2

Effluent BOD5 (mean concentration 4.6 mg/L) and TSS (mean concentration 2.8 mg/L) during 6-month standard performance period.

At startup, the CBT experienced a stabilization period of about 2 weeks. The effluent TSS was about 18 mg/L at startup but then remained between 1 and 4 mg/L throughout the test duration (Figure 2). Similarly, effluent BOD5 increased to about 44 mg/L at startup then decreased to less than 5 mg/L and remained thus for the rest of the test period (Figure 2). Similarly, DO of ML was above 4 mg/L at the start and gradually decreased and fluctuated between 1.0 and 3.0 mg/L with an average concentration of 2.3 mg/L ±1 after 2 weeks (see SI, Figure S2, online at http://www.iwaponline.com/wst/071/172.pdf).

After 6 months of operation, DO and ML solids concentration were approximately 1.5 mg/L and 11,000 mg/L, respectively. Concomitant with ML concentration increase, effluent TSS concentration decreased substantially over time (Figure 2). Although effluent TSS concentration decreased with an increase in MLSS concentration, the low DO level and high sludge accumulation in the tank after 6 months of testing is an indication that sludge wasting is necessary approximately twice per year if the unit is operated at its design capacity. An immediate increase in DO level to approximately 4.0 mg/L was observed in the ML after approximately 80% of the accumulated sludge was removed (new ML was 2,000 mg/L).

Although there was a large fluctuation in influent TSS concentration (between 150 and 570 mg/L) (see SI, Figure S1), effluent TSS levels were relatively stable and average TSS removal rate was 98.9%. The average influent and effluent TSS concentrations were 241.5 ± 67.5 mg/L and 2.7 ± 2.8 mg/L, respectively (Table 1). The OESIS-750 unit also experienced a gradual decrease in DO from about 5 mg/L at the beginning to about 2.5 mg/L at the end of the 6-month testing period (Babcock et al. 2004). The average effluent TSS concentration reported was 13.1 ± 6.9 mg/L. Similar to the CBT, the OESIS-750 unit also required sludge removal after 6 months. Large variations of DO were observed within the CBT due to the cyclic aeration (aerobic/anoxic) process. Both the CBT and the OESIS-750 units easily achieved the NSF40 Class I TSS and BOD requirements.

Stress tests period

Temperature, pH and DO in the ML were not affected during all four test periods (see SI, Figure S3, online at http://www.iwaponline.com/wst/071/172.pdf). During the stress test I, the effluent TSS increased to approximately 4 mg/L but then returned to about 1 mg/L (Figure 3) at the end. During the stress test II, the effluent TSS increased to about 4.6 mg/L but returned to 1 mg/L after 5 days of the test (Figure 3). A similar slight rise in effluent BOD5 was observed (increase from 2.5 to 4.5 mg/L) but decreased to below 2 mg/L approximately 5 days after the test. Babcock et al. (2004) reported that the OESIS-750 also behaved similarly in terms of TSS with stable concentrations in both of the first two stress tests. However, it experienced a BOD5 increase to 10 mg/L followed by a recovery period of approximately 7 days in stress test I while no change of BOD5 occurred in stress test II.

Figure 3

Effluent BOD5 and TSS during series of four stress tests.

Figure 3

Effluent BOD5 and TSS during series of four stress tests.

Stress test III caused an increase in both TSS and BOD5 concentrations to about 11 mg/L and 7.3 mg/L, respectively (Figure 3). However, the CBT recovered rapidly and the effluent TSS concentration fell to 2.3 mg/L on the second and then to less than 1 mg/L on the third day after the test. Similarly, the effluent BOD5 decreased to 1.2 mg/L 2 days after the test. The OESIS-750 unit experienced no change in effluent TSS but effluent BOD5 increased to slightly above 30 mg/L for 1 day but then recovered quickly to approximately 15 mg/L after 3 days (Babcock et al. 2004). Stress test IV also caused an increase in effluent TSS and BOD5 concentration to approximately 5 mg/L and 8.5 mg/L, respectively, but started to decrease soon after the test. The BOD5 of 8.5 mg/L decreased to 3.7 mg/L 3 days after the stress period ended. During stress test IV, the OESIS-750 experienced a slight increase in effluent TSS only for 1 day. However, effluent BOD5 increased considerably to 72.5 mg/L 1 day after the test ended. The BOD5 concentration decreased to 52.5 mg/L, 31.3 mg/L and then to below 30 mg/L after the second, third and fourth day of the test, respectively.

Among the four stress test periods, the fourth period affected the treatment the most in both systems. Although the OESIS-750 unit did not handle stress tests 3 and 4 as well as the CBT, the effluent still complied with NSF40 Class I standards throughout testing. The CBT was more stable compared to the OESIS-750 unit. Table 1 shows a summary of results of both units during the standard performance test.

Nutrient removal

The average influent total N and P concentrations were 28.1 mg/L and 4.5 mg/L, respectively. The CBT performed very well in terms of nutrient removal, achieving an average 81% for N (see SI, Figure S4) and 51% for P (see SI, Figure S5), respectively, during the standard performance test period (Figures S4 and S5 are both available online at http://www.iwaponline.com/wst/071/172.pdf). The effluent contained only 3.1 mg/L of ammonia, approximately 2.1 mg/L of nitrate, and 2.2 mg/L of phosphorus. Effluent ammonia concentrations were high during the startup period (until the ML developed) and then dropped to less than 3 mg/L for most of the time during the standard performance period (data not shown). The OESIS-750 unit achieved only 19% N removal and 17% P removal (Babcock et al. 2004). The results show that the anoxic/anaerobic cycles in the CBT allow enhanced nutrient removals, especially nitrogen. Table 2 shows a summary of nutrient removal results for both the CBT and the OESIS-750 units.

Table 2

Nutrient removal efficiency of CBT 0.8KF-210 and OESIS-750 ATUs

  CBT 0.8KF-210 OESIS-750 
Nutrients Average value Standard deviation Average value Standard deviation 
Nitrogen 
 Influent total nitrogen (mg N/L) 28.1 7.3 23.1 4.4 
 Effluent total nitrogen (mg N/L) 5.2 3.6 18.6 2.9 
 Nitrogen removal (%) 81  19  
Phosphorus 
 Influent total phosphorus (mg P/L) 4.5 1.4 4.6 1.5 
 Effluent total phosphorus (mg P/L) 2.2 1.3 3.8 1.5 
 Phosphorus removal (%) 51  17  
  CBT 0.8KF-210 OESIS-750 
Nutrients Average value Standard deviation Average value Standard deviation 
Nitrogen 
 Influent total nitrogen (mg N/L) 28.1 7.3 23.1 4.4 
 Effluent total nitrogen (mg N/L) 5.2 3.6 18.6 2.9 
 Nitrogen removal (%) 81  19  
Phosphorus 
 Influent total phosphorus (mg P/L) 4.5 1.4 4.6 1.5 
 Effluent total phosphorus (mg P/L) 2.2 1.3 3.8 1.5 
 Phosphorus removal (%) 51  17  

Oil and grease, turbidity, and UVT254

The average oil and grease concentration in the influent was 31.4 mg/L (max 67.5, min 15.5). Out of the 11 measurements, a detectable concentration of oil and grease in the effluent occurred only once and was 1.8 mg/L (detection limit: 1.0 mg/L). Effluent turbidity was high (10–12 NTU) during the startup period and then decreased and stabilized at less than 2 NTU for nearly the entire standard performance test period (see SI, Figure S6, online at http://www.iwaponline.com/wst/071/172.pdf). The effluent turbidity increased somewhat during stress tests (2–3.5 NTU), but recovery was rapid. The unit produced such a low turbidity effluent that it is suitable for direct media filtration in order to produce unrestricted use recycled water (without any pretreatment). In contrast, the OESIS-750 unit produced effluent with higher turbidity (6.3 NTU average) that was less than 5 NTU only 50% of the time, indicating that the effluent would have to be chemically coagulated prior to filtration (Babcock et al. 2004). Effluent UVT254 was between 60 and 70% during startup and then increased to between 70 and 80% (average 74%) for the remainder of the tests (see SI, Figure S7, online at http://www.iwaponline.com/wst/071/172.pdf). These UVTs are much greater than the 55% minimum required post filtration to produce R-1 water. In contrast, the OESIS-750 unit produced effluent with lower UVT254 (between 30 and 54%; average 40%) (Babcock et al. 2004) further indicating the need for chemical coagulation prior to media filtration in order to produce R-1 recycled water. No odor, oily film, or foam was present in the effluent chamber of either unit (treated water).

DISCUSSION AND CONCLUSIONS

The CBT performed very well during the entire standard performance and stress test periods, which included no maintenance and met NSF40 Class I requirements as well as the NSF-245 requirements for nitrogen removal (the same as CZARA and CWDA standards) and the turbidity requirements in NSF-350. The condition of the unit was very stable and recovered very quickly after the stress tests. Suspended solids, settleable solids, and BOD5 decreased to acceptable levels without the production of offensive odors, oily film, or foam. To ensure acceptable effluent quality at all times, regular inspections and maintenance of the CBT should be conducted every 6 months. The effectiveness of the CBT in removing nitrogen (81% removal rate) shows the benefit of having cyclic aerobic and anoxic conditions in ATUs for nitrification and denitrification to occur. The CBT nitrogen removal capability is significantly better than the OESIS-750, which did not meet the NSF-245, CZARA and CWDA standard of 50% N removal. The CBT effluent was found to have very low turbidity (<2 NTU) and a very high UVT254 (>70%) making it highly suitable for recycling applications. In contrast, the effluent from the OESIS-750 unit (with higher turbidity and lower UVT254) would require more treatment in a recycling application. Unrestricted use recycled water can be used for watering lawns, gardens, in fountains, for flushing toilets, and for many other uses. Onsite water reuse in this way can offset potable water use and thereby contribute to drinking water sustainability. Both the CBT and OESIS-750 units performed very well and met all NSF40 criteria for a Class I effluent; therefore, both units are favorable for residential use.

ACKNOWLEDGEMENT

The authors would like to thank International Wastewater Technologies for sponsoring the research.

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