Current Performance
Overview System Highlights Current Performance
Litigation System Improvements Glossary & References

Current Performance

The Arcata Wastewater Treatment Plant's design utilizes passive, natural processes to treat wastewater. As these biological processes are dependent upon the surrounding environment, fluctuations in weather and environmental conditions can affect performance. Under normal operating conditions the Arcata Wastewater Treatment Plant complies with permit regulations. Violations of discharge permit parameters tend to occur during periods of wet weather, when high levels of infiltration can increase the volume of inflow to more than five times the design annual flow (AMIC "A Natural System").

Discharge water entering Humboldt Bay from Outfall 001.

Figure 1: Effluent discharge into Humboldt Bay from Outfall 001.


The Arcata Wastewater Treatment Plant is regulated by the State of California North Coast Water Quality Control Board. The Water Board administrates the NPDES discharge permit program in California on behalf of the EPA, overseeing compliance with all water quality statutes. Principle statues include the Federal Clean Water Act, and the State of California Porter-Cologne Act.

Treated effluent is discharged into Humboldt Bay at Outfall 001 (Figure 1). The outfall is located on an enclosed bay and estuary. The Arcata Wastewater Treatment Plant is subject to more stringent water quality regulations because of this outfall location.


The Arcata Wastewater Treatment Plant operates under two NPDES discharge permits. One permit regulates Outfall 001, where the treated water is discharged into Humboldt Bay. The other permit regulates Outfall 002, where the effluent is discharged from the Arcata Wastewater Treatment Plant into the Arcata Marsh and Wildlife Sanctuary. Tables 2 and 3 below summarize the current effluent limits for Outfall 001 and Outfall 002 (Kulhman, 2007).

Table 2: Effluent limits for discharge to Humboldt Bay (OUTFALL 001). Source: ACL Complaint No. R1-2007-0064 (Kulhman, 2008)

Constituent Units Monthly Average Weekly Average Daily Maximum
BOD5 mg/L 30 45 60
" lbs/day 575 863 1151
Suspended Solids mg/L 30 5 60
" lbs/day 575 863 1151
Settleable Solids ml/L 0.1 - 0.2
Total Coliform MPN/100ml 14 - 43
pH S U Not less than 6 nor greater than 9
Copper mg/L 2.8 - 5.7
Zinc mg/L 47 - 95
Cyanide mg/L 0.5 - 1.0
2,3,7,8-TCDD TEQ pg/L .014 - .028

Table 3: Effluent limits for discharge to Arcata Marsh and Wildlife Sanctuary (OUTFALL 002). Source: ACL Complaint No. R1-2007-0064 (Kulhman, 2008)

Constituent Units Monthly Average Weekly Average Daily Maximum
BOD5 mg/L 30 30 60
Suspended Solids mg/L 30 45 60
Settleable Solids ml/L 0.1 - 0.2
Total Coliform MPN/100 mL 23 - 230
pH Standard Units Not less than 6.0 nor greater than 9.0

Performance Under Normal Conditions

The Arcata Wastewater Treatment Plant generally complies to all regulations under normal operating conditions. The EPA Enforcement and Compliance History Online (ECHO) database contains detailed effluent reports submitted quarterly by the Arcata Wastewater Treatment Plant.

The existence of two discharge permits necessitates the mixing of effluent to conform with the discharge criteria contained in Tables 2 and 3. After the wastewater has passed through the enhancement marshes, the BOD and TSS levels are usually under 10 mg/L. Wastewater exiting the treatment wetlands generally has BOD/TSS levels of 40-60 mg/L. Effluent from the treatment wetlands and the enhancement marshes are then combined to obtain an effective BOD/ TSS concentration of 30 mg/L so that the water discharged into both the Arcata Marsh and Wildlfe Sanctuary and into Humboldt Bay complies with permits (AMIC, "A Natural System").


The EPA discloses that for 9 of the past 12 quarterly reports (January 2005- December 2007), the Arcata Wastewater Treatment Plant was found to be in violation of discharge permit limits (EPA, "Facility Report.", 2008). Figure 2, from the NPDES effluent report for January 2005- December 2007, displays violations of TSS levels. For this report, in only one instance concentration limit (mg/L) criteria were exceeded. The remainder of the violations occurred when the total quantity (lbs/day) of TSS were exceeded.

Infractions of BOD and TSS mass limits (lbs/day), and the 85% Percent Removal efficiency criteria constitute the majority of the Arcata Wastewater Treatment Plant violations (Kulhman, 2007). Mass limits place a cap on the total amount (by weight, in lbs) of a pollutant that can discharged each day. Percent Removal requirements stipulate that, based on influent concentrations, 85% of pollutant levels must be removed before discharging the water. These criteria are evaluated based on a 30 day average.

Report graph of TSS discharge levels January 2005 through December 2007

Figure 2: NPDES Report: Total Suspended Solids for Outfall 001 (EPA, "NPDES.", 2008)

Rainy Season Infiltration Affects Performance

The City of Arcata's wastewater treatment collection system is susceptible to large volumes of infiltration and inflow during the rainy season due to older infrastructure. During a large storm events, peak flows on the order of 16.5 MGD or more can be reached. Such storm inflows are over five times larger than the yearly average of about 3 MGD (AMIC "A Natural System"). Inflows exceeding 5 MGD must be routed directly to the oxidation ponds to avoid flooding the system. During a seasonal peak flow of 16.5 MGD, this means that over 65% of the influent will not undergo primary treatment.

Mass Limits Exceeded

Dave Couch, an operator at the Arcata Wastewater Treatment Plant, attributes the cause of most mass limit violations to such storm events, particularly at the beginning of the rainy season. During the warmer, dry summer months a "large standing crop of algae" accumulates in the oxidation ponds and treatment wetlands. When the rainy season begins, the increased flows due to stormwater infiltration flush out this algae (Couch, 2008). The increased volume of water surges through the Arcata Wastewater Treatment Plant at a rapid rate and is not retained in the treatment wetlands long enough to effectively eliminate BOD and TSS.

Percent Removal Difficulties Due to Dilution

The rainy season also poses difficulties in compliance with the 85% Percent Removal requirement. This component requires an over-arching 85% removal of all criteria pollutants from the influent before discharge.

Under normal operating loads, influent concentrations entering the Arcata Wastewater Treatment Plant are around 200-300 mg/L (Couch, 2008). The 85% removal would then require effluent discharge levels to be 30 mg/L or lower, which is in line with plant performance under normal conditions. However, when peak flows occur during the rainy season, stormwater dilutes the concentration of influent to around 80 mg/L (Couch, 2008). The effluent would then need to be treated to a concentration of 12 mg/L to meet the percent removal requirement.