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
        
