CA NDZ: Vessel Sewage Data Analysis for LOGVs

September 20, 2011

Data QC for Large Oceangoing Vessels (LOGVs): 

EPA used US Coast Guard (USCG) data on LOGV vessel port calls, and Chamber of Shipping of America (CSA) data on LOGV size and sewage treatment to determine the volume of sewage 

annually generated in State marine waters that might be held, and the volume that might be discharged into State marine waters, if EPA promulgated a rule that prohibited LOGVs with holding 

tanks from discharging sewage to State marine waters except for sewage generated within State waters that exceeds the LOGVs maximum holding capacity. EPA also used this information to 

calculate the volumes of sewage that might be held and discharged under the rule proposed in the NPRM.

1. USCG vessel call data from 2010 was used to determine length of port calls for LOGVs entering California waters. Time entries were converted to hours. 682 passenger vessels were not 

included. Nine entries were not included because they were for 2011, and 15 entries were not included because they did not include departure dates. A total of 8,751 LOGVs >300 GRT were 

assessed.

2. CSA data from 2010, which includes information on the size, crew, sewage holding tank capacity, sewage generation rate and MSDs for LOGVs operating in US waters, was used to determine 

median daily sewage generation and median holding capacity for the 587 surveyed vessels >300 GRT. The median rate of sewage generation for the surveyed LOGVs is 397 gallons per day. 

Volume generated per day was reported by each vessel in the CSA survey. This translates to crew member sewage generation rate of 16 gallons per day (almost twice the rate estimated for the 

NPRM based on large passenger ship data). 

Vessels were categorized by days of holding tank capacity in the following increments: 0, 0-1, 1.1-2, 2.1-3, 3.1-4, 4.1-5, and greater than 5.16 days (the median length of the longest port call 

category from the USCG data (4.1-731 days)). The final category, median holding capacity >5.16 days, was used as the cutoff since most vessels with this amount of holding capacity are unlikely 

to discharge while in port. The median tank capacity for each holding capacity strata was identified. 
Table 1. Median Holding Capacity

CSA LOGV Data

 

Holding 

Capacity 

(days)

Vessel 

Count Percent

Median 

(days)

0 224 38% 0

0.1-1 33 6% 0.8

1.1-2 42 7% 1.5

2.1-3 16 3% 2.5

3.1-4 26 4% 3.6

4.1-5 43 7% 4.7

>5.16 200 34% 12.2

 

3. Port calls were stratified into five categories depending on length of stay in the following increments: 0-1, 1.1-2, 2.1-3, 3.1-4, and 4.1-73 days. The median time in port was determined for 

each of the five categories. The amount of sewage generated for each port call category (per visit) was calculated by multiplying the median length of port stay by the median vessel sewage 

generation rate of 397 gallons/day obtained from the CSA data. The total annual volume of sewage generated was determined for each port call category by multiplying the sewage generated 

in each category by the respective number of port call reports for that category. The sum of the five port call categories was approximately 6.0 million gallons of sewage generated for 2010. 

According to the CSA data, 38% of vessels have zero holding capacity. The number of reports for each port call length category was multiplied by 38% to estimate the length of port calls by 

vessels with zero holding capacity and the product was multiplied by the estimated sewage generation for each length of port call to estimate how much sewage might be discharged by vessels 

with zero holding capacity over a year (result ~ 2.3M gallons/year). The percentage of vessels in each of the holding capacity categories was calculated. For each category, the volume of sewage 

that could be held in holding tanks, and the volume of sewage that exceeded holding tank capacity was calculated. As expected, increasing holding capacity resulted in reduced discharge and 

increased the volumes of sewage that could be held. For each category, the volume of sewage that would be discharged because it exceeded vessel holding capacity was determined by 

subtracting the holding capacity days from the length of port call days and multiplying the difference by the median generation rate of 397 gallons/day, then multiplying the product by the 

percentage of port calls with a specific holding capacity. For example, if a vessel is in port for 2.5 days and only has 0.8 day holding capacity, the vessel is expected to discharge for 1.7 days (2.5 -

0.8 = 1.7). Multiplying 1.7 days by the 397 gallon/day median discharge rate yields the amount of sewage discharged over the course of the 2.5 day port call. Since 6% of LGOVs are in the 0.8 

day median holding capacity category, 6%(0.06) is multiplied by the number of 2.5 day port calls (1,468) to determine the number of port calls where a discharge of 1.7 days will occur. The 
calculation for the category of LOGVs with 2.5 day port calls and 0.8 day median holding capacity is (2.5 - 0.8 days) X 397 gallons/day = 674.9 gallons/call X (6% X 1,468 port calls = 88) = 59,445 

gallons discharged /year. This process was repeated for each port call length and each holding capacity category as shown in Table 2. 

Table 2. Vessel Sewage Generation Analysis

Port Call Values LOGV Discharge and Holding Volumes by Holding Capacity Category (gallons)

Port 

Call 

Days

2010 

Vessel 

Call 

Reports

Percent 

of 

Total

2010 

Port 

Call 

Days 

Median

Gallons 

generated 

based on 

port call 

days X 

*397 g 

(median)

Total 

annual 

volume 

generated 

by length 

of port 

call 

category

Volume 

DISCHARGED 

for vessels 

with zero 

holding 

capacity 

(38% of 

vessels)

Volume 

DISCHARGED 

for vessels 

with 0.8 day 

median 

holding 

capacity (6% 

of vessels)

Volume 

HELD 

for 

vessels 

with 0.8 

day 

median 

holding 

capacity 

(6% of 

vessels)

Volume 

DISCHARGED 

for vessels 

with 1.5 day 

median 

holding 

capacity (7% 

of vessels)

Volume 

HELD 

for 

vessels 

with 1.5 

day 

median 

holding 

capacity 

(7% of 

vessels)

Volume 

DISCHARGED 

for vessels 

with 2.5 day 

median 

holding 

capacity (3% 

of vessels)

Volume 

HELD 

for 

vessels 

with 2.5 

day 

median 

holding 

capacity 

(3% of 

vessels)

Volume 

DISCHARGED 

for vessels 

with 3.6 day 

median 

holding 

capacity (4% 

of vessels)

Volume 

HELD 

for 

vessels 

with 3.6 

day 

median 

holding 

capacity 

(4% of 

vessels)

Volume 

DISCHARGED 

for vessels 

with 4.7 day 

median 

holding 

capacity (7% 

of vessels)

Volume 

HELD 

for 

vessels 

with 4.7 

day 

median 

holding 

capacity 

(7% of 

vessels)

Volume 

DISCHARGED 

for vessels 

with >5.16 

day holding 

capacity 

(34% of 

vessels)

Volume 

HELD for 

vessels 

with 5.16 

day 

holding 

capacity 

(34% of 

vessels)

0 - 1 3,559 42% 0.63 248 883,077 335,569 0 53,408 0 62,310 0 26,704 0 35,606 0 62,310 0 302,648

1.1 - 2 2,045 24% 1.46 579 1,183,699 449,806 32,052 38,970 0 82,973 0 35,560 0 47,413 0 82,973 0 403,010

2.1 - 3 1,468 17% 2.50 993 1,456,990 553,656 59,445 27,974 40,796 61,194 0 43,710 0 58,280 0 101,989 0 495,377

3.1 - 4 843 10% 3.46 1,373 1,157,292 439,771 53,373 16,064 45,870 35,140 9,618 25,100 0 46,318 0 81,057 0 393,707

4.1 -

731 656 8% 5.16 2,049 1,343,829 510,655 68,129 12,501 66,723 27,345 20,782 19,532 16,251 37,502 8,386 85,682 0 456,902

8571 6,024,887 2,289,457 213,000 148,917 153,388 268,962 30,401 150,606 16,251 225,119 8,386 414,011 0 2,051,643

6.0M

Sum 

Discharged Sum Held

2,710,883 3,259,259

2.7M 3.3M
4. Data Analysis for the Proposed Rule (9/2/10), prohibiting discharges to State marine waters from all LOGVs >300 GRT with 2 days or more holding capacity: Vessel holding tank and port call 

categories are stratified the same as above. Based on CSA data, 49% of vessels had 2 days or more holding capacity and 51% had less than 2 days holding capacity. Using this data, including the 

higher sewage generation rate, annual 2010 vessel sewage generation for each port call category was multiplied by 49% and 51%. The result indicates that the 49% of vessels with 2 days or 

more holding capacity would be prohibited from discharging approximately 2.9M gallons per year and the 51% of vessels with less than 2 days holding capacity, not subject to the proposed rule, 

would not be prohibited from discharging approximately 3.1M gallons per year. 

Table 3. Vessel Sewage Generation Method Used for Proposed Rule

Port Call Values Large Oceangoing Vessel Sewage Volumes (gallons)

Port Call 

Days

2010 

Vessel 

Call 

Reports

Percent 

of 

Total

2010 

Port 

Call 

Days 

Median

Gallons 

generated 

based on port 

call days X 

*397 g 

(median)

Total 

annual 

volume 

generated 

by length of 

port call 

category

Sewage 

generated by 

vessels with 

less than 2-

days holding 

capacity

Sewage 

generated by 

vessels with 2-

days or more 

holding 

capacity

0 - 1 3,559 42% 0.63 248 883,077 450,369 432,708

1.1 - 2 2,045 24% 1.46 579 1,183,699 603,687 580,013

2.1 - 3 1,468 17% 2.50 993 1,456,990 743,065 713,925

3.1 - 4 843 10% 3.46 1,373 1,157,292 590,219 567,073

4.1 -

731 656 8% 5.16 2,049 1,343,829 685,353 658,476

6,024,887 3,072,693 2,952,195

6.0M 3.1M 2.9M