2.1 Material Flow Analysis of Plastic Waste (2021)
Material flow analysis both in Jakarta and Bandung was developed to study the plastic waste before categorized as the NRPW. Result shows the plastic waste generation in Jakarta was 399,691 ton/year from 10,534,339 people , 0.7 kg/capita/day of municipal solid waste (MSW) generation rate , and 14.85% of plastic waste in MSW . Jakarta’s plastic waste composition consisted of 5.85% of PET and 9.00% of mixed plastic . Meanwhile, for Bandung with population of 2,584,252 people , the MSW generation rate and the percentage of plastic waste are 0.63 kg/capita/day  and 22.45%  of plastic fraction. This plastic composition consisted of plastic bottles, plastic glass, plastic wraps & multilayered, plastic containers, plastic bags, and diapers. Therefore, plastic waste generation in Bandung is 133,409 ton/year. Figure 1 contains the material flow of plastic for both areas.
In terms of plastic waste collection at the source, 44,281 ton/year (11%) was uncollected in Jakarta, while 326,829 ton/year (82%) and 28,581/year ton (7%) of them were collected by collection services and informal value chain. In Bandung, 12,512 ton/year (10%) of plastic waste was not collected. Then 93,973 ton/year and 26,924 ton/year were collected by collection services and informal value chain (70 and 20%, respectively). Informal value chain is related to the informal waste pickers (e.g. scavengers) commonly collecting the waste at the streets and disposal sites. Managing after the collection, the collection services diverted to the sorting facility for recovery about 106,043 ton/year and 12,300 ton/year in Jakarta and Bandung, respectively. Jakarta’s higher percentage was supported by waste sorted for energy with a capacity of about 100 ton/day  which also improved the realization of 34% of plastic waste possible to be recycled as the energy form . Respectively, 95,926 ton/year and 36,500 ton/year were sorted from pre-treatment facilities for recycling and energy from wastes in Jakarta. On the other hand, 38,436 ton/year of plastic waste was sorted for recovery (e.g. mechanical recycling) without waste to energy in Bandung. Further, a total of 211,309 ton/year of plastic waste was transported by Jakarta’s municipality to final disposal in Bantar Gebang, Bekasi City. In Bandung, 78,570 ton/year of plastic waste was disposed in Sarimukti, West Bandung District.
From both cities, leakage is based on the uncollected waste and residual term of management stages. Jakarta was responsible for 28,472 ton/year of plastic waste entering the waterway across five cities. On the other hand, Bandung discharged the plastic waste of approximately 10.945 ton/year to the waterway. Most of the plastic waste leaked to the waterway was sourced from the direct disposal where Jakarta acquired 57.6% and Bandung at 56.08% of the total waterway leakage, delivering the first phase of investigation on the human activity as the contributor of litter . Demonstrate the main contribution of plastic debris into the waterway is the land-based source    through direct littering due to inadequate waste management system.
2.2 Quantity and Composition of Non-Recyclable Plastic Waste Leakage
Plastic waste leakage from an uncollected waste was the largest contributor in both areas, yet we also considered the leakage from other waste management activities. During the collection activities, there were 9,477 ton/year from collection services and 257 ton/year from the informal value chain in Jakarta. On the other hand, 3,101 ton/year and 215 ton/year were leaked sequentially from the collection services and informal value chain in Bandung. Apart from collection, disposal facilities in both areas added the leakage by 4,248 ton/year in Jakarta and 7,910 ton/year in Bandung. Despite of higher disposed plastic waste in Jakarta than Bandung, the leakage from disposal facilities were less since we put lower leakage potential levels from environmental hazards (e.g. flooding or landslides) and fencing in the WFD tool. For example, we found the treatment and environmental management were adequate in Jakarta’s final disposal and treatment facility, which is called TPST Bantar Gebang . In terms of sorting facilities, formal sorting (e.g. waste banks) in both Jakarta and Bandung do not contribute to plastic leakage since the waste banks sorted out the valuable plastics at the source. As a results, the amount of reject from waste banks are very low and back to the formal collection system. On the contrary, 1,941 ton/year and 573 ton/year of plastic waste were leaked from informal sorting in Jakarta and Bandung, respectively.
When the plastic waste entered the environment, 4,622 ton/year was burnt and 25,770 ton/year was retained on land, while 13,403 ton/year and 16,409 were respectively entered drainages and the Jakarta’s River systems. Further, 1,340 ton/year was retained on the drainage systems, while the rest of 12,063 ton/year was discharged into the waterway. Hence, a total of 28,472 ton/year of plastic waste was leaked into the waterway in Jakarta. When disposable diapers took into account, the number increased to 40,880 ton/year since its responsible for about 30%  of the total waterway waste leakage composition in Jakarta. We also included disposable diapers in this study since they were considered as a NRPW. In Bandung, it was counted that 1,071 ton/year, 11,990 ton/year, and 5,113 ton/year of plastic waste were leaked into the air through open burning, retained on land, and entered the storm drains, respectively. Moreover, plastic waste was littered directly into the river about 6,139 ton/year. Therefore, 10,945 ton/year of plastic waste was leaked into the waterway in Bandung. In other words, the ratio of plastic waste littering per person in Bandung and Jakarta are, respectively, 3.88 kg/year and 4.24 kg/year.
From the total plastic waste leakage into the waterway, we estimated the amount of NRPW in both cities, which were 37,995 ton/year (9.51% of total plastic waste generation in the city) in Jakarta and 10,636 ton/year (7.97% of total generated plastic waste in the city) in Bandung. Plastic bottles and plastic cups are not included as non-recyclable plastic due to the availability of the recycling facilities and their relatively higher economic value which is different from the disposable diapers, multilayered plastic packaging, plastic bags, and other plastic. In Jakarta, plastic bag was identified as the most NRPW in the waterway with 20,777 ton/year, the fact that waste is frequently disposed of inside plastic bags. This followed by 12,408 ton/year of disposable diapers, 3,944 ton/year of plastic packaging, and 866 ton/year of other plastic respectively. Meanwhile, in Bandung, plastic bags and diapers were discovered as the most NRPW in the waterway with the same amount of 3,477 ton/year. Further, it is followed by 2,215 ton/year of plastic packaging and 1,468 ton/year of other plastic respectively. Based on our model, the composition of NRPW in the waterway for both Jakarta and Bandung is shown in Fig. 2.
2.3 Potential Leakage Hotspot Identification
With the waste generation from the overall population in each city, Jakarta has discharged 37,995 ton per year, and Bandung discharged 10,636 ton per year to the waterway. Based on the identification using Waste Flow Diagram tool, both cities indicated the most leakage from the overcapacity of waste management facilities. Therefore, we consider the amount of the hotspot leakage based on the residual term and topographical feature related to the location of the facilities. On the other hand, the leakage also implied from the uncollected term, which lead to the possibilities of direct disposal of plastic waste to the water system–which regarded as the NRPW. Therefore, limited by the residual and direct disposal, both cities respectively implied the possible amount of water leakage distribution in Fig. 3 in the accumulated ton in an annual basis.
We found that over 225 hotspots which came from the waste management facilities in Jakarta, indicating 24.3% of the waste management are responsible for the potential leakage source which contributes to the river pathway. Due to the method of facilities assessment and its proximity to the river pathway, spatial distribution indicates 39.12% of the facilities are in the high-zone proximity (0 – ≥100 m) and 7% of the facilities obtained fully potential to the leakage on an annual basis. Most of the contributor of the facilities prominent in Dipo types (transfer station in the community level), which also depends on the zonal coverage. The second contributor were TPS 3R (3R station for material recovery), which exceeded 61.3 ton per year; indicating how the concept of TPS 3R needs to be improved in Jakarta . On the other hand, we assumed the management of waste bank has effectively managed with zero leakage based on the calculation and assessment through the spatial approach. This Implied that the collection at the source and the current management are in the good prospect of Jakarta waste management .
Another source of the leakage in Jakarta area was from the plastic waste generation in the residential area, which was also responsible to the 36.76% of the contribution in the final disposal . Limited to the population settled in the riverbank area, approximately 2,080,076 population (19.7% of total population in Jakarta contributes 28.944 ton/year of plastic waste at the source. We found the potential of the direct disposal term due to the cramped population at the wards in slum categorized exceeding 87,696 population or 42% of the population settled on the proximity of the river pathway in between 0 to 300 m buffer area from river centroid across the city.
In Bandung, there were 122 waste management facilities contributing to the leakage in Bandung. Based on the same method for the residual leakage, we found 25 facilities are high in leakage probability, where 14 of the facilities are located close to the riverbank area (100-meter proximity). Although the leakage hotspots in Bandung are half of the Jakarta, the leakage exceeds 120.85 ton in annual basis for the minimum leakage (Jakarta is 61.3 ton/year). Based on an assessment on the spatial range, we found 25 facilities potentially contributing to the waterway as the possibilities to the leakage. In short, Bandung is comparable with Jakarta in term of leakage to the waterway according to its proportion; where Jakarta contributed the leakage of 61.3 ton in annual basis from 24.3% possible sources, and Bandung is 120.85 ton in annual basis from 20.5% of possible sources of leakage. Therefore, its pathway as the mobilization of the leakage material needs to be concisely constructed from the water pathway.
Overlaid with the condition on the topographic condition and hydrological condition, the morphometric parameters utilized over 31.7% of the waterway in Jakarta are considerably high on the plastic leakage and 18.3% in Bandung. Correlating with the elevation, Bandung has higher differences in the elevation on the highland area, considering how the water flow are likely high in the mountainous area  which considered the higher chance of plastic waste leakage to shift to the other area. Comparing with Jakarta, which is in the coastal area and considerably flat, causing higher chance of access to the waterway.
Type of river distribution is also considered where both cities have different characterization of the mainstream; Jakarta has several mainstreams which likely non-concentrated area of the flow while Bandung has only one mainstream which all the streams in Bandung is from Citarum River in the Southern part, considered how the flow vary in terms of the accumulation of runoff to Citarum River.
2.4 Plastic Leakage Pathway and Seasonal Changes
Wrapping up the conclusion of flow characteristics judged based on the morphometric analysis towards the digital elevation model comprehend the full picture of the leakage mobilization in the waterway, shown in Fig. 4. The result shows that rainy season has fully implication on higher accumulation and less hotspot area. Therefore, an assumption on the quantification method in the hydrological characterization is applied to improve the retained material in the low runoff. According to the spatial distribution of the river network and the leakage source hotspot, 67 hotspots contemplate with river networks in Bandung uptakes the leakage contribution over 82.18 ton per year.And in Jakarta we found 742 locations contributes over 34.41 ton per year across the riverbank area.
Based on the distribution of river network, Jakarta exceedingly obtained 83 networks across the 5 cities and divided to 7 mainstreams which directly flow to the ocean from Jakarta Bay. In Fig. 4 (A), Jakarta obtains the leakage distribution more in the transboundary rivers (Western part directly adjacent with Tangerang and Eastern part with Bekasi) where potentially 442.58 ton in annual basis was input to the Mookervart River in West Jakarta and 165.51 ton in the Cakung River adjacent with Bekasi in North Jakarta.
According to the slum area assessed from BPS and KOTAKU , it shows the sparsely dense in Northern area of Jakarta. The slum area distribution is related to the leakage pathway since there are more plastic waste leakage to the river near the slum-graded wards. We found the higher leakage in the streams to Tanjung Priok port area of approximately 507.56 ton. This causes more evidence of high intensity of land and sea-based activity contributes on the plastic releases in annual basis in the same case with Jambeck’s study (2015). It is also found how the intense activity on land causing higher intensity of leakage flow in the waterway . Considering how the slum area also regarded to the higher population density lived in the riverbank area with poor-managed waste management  , proof the direct disposal and low-managed of waste management are still existing in developing country. It is also proven by the leakage load which is higher in the Northeast part of Jakarta  as slum area and higher activity. On the other hand, direct disposal also considered happened due to unrecorded and unidentified illegal dumping which also same in the African case studies according to Verster and Bouwman  ; prevalent to the low-managed area especially in slum categorized area, in extent need to be addressed as the leakage.
The distribution of plastic leakage pathway in Bandung (Fig. 4.B) shown the concentrated leakage found in the Southern part of the city which adjacent to Bandung Regency, lead the waterway to the mainstream of Citarum River (represent in dark-colored delineation line in Fig. 4 part B). As seen in in Fig. 4, the leakage pathway directed to the mainstream and obtained the higher amount of input to the river towards the tributaries in Citarum River. Here we found the higher concentrated input to the waterway in Cikapundung Kolot based on the surveyed in November 2021. Correlating with the classified river flow in Fig. 3B, the continuous low classified flow in the directions to Citarum defines how the lower flow lead to higher accumulation, causing the higher result input in the waterway. Thus, around 108.77 ton of plastic waste likely was forecasted to input to Citarum as the mainstream in an annual basis.
Comparing side-by-side, the leakage source hotspot in Bandung area (Fig. 3.B) with the leakage pathway in the river pathway (Fig. 4 part B),Western part of Bandung which also adjacent to the Cimahi City detected high concentrated leakage source hotspot.. This condition implies due to the wider zone of slum regarded area. The topographic condition is also affected the result from morphometric analysis.
Both Jakarta and Bandung condition of accumulation provided with the runoff, correlated with rainfall rate (see Appendix Data S2). We found that rainfall and flash flood identification (from the morphometric analysis and peak runoff correlation) considered where most of the mainstreams are responded highly to the leakage. Contextualizing the data from satellite-based precipitation   and national weather station observation , the peak rainfall on both cities is high in period of November to May (see Appendix Data S2 Fig. S3), initializing the possibility of flash flood and fluctuations in plastic leakage to the river pathway.
Aforementioned higher rainfall rate in November to May period and respond to the higher input to the pathway, we found Jakarta exceedingly high in February which can be shown in Fig. 5. Both cities were high in the transitional period in May where potentially 826.52–1114.82 ton of plastic waste is discarded. We predicted that higher rainfall rate which causing the flash flood has correlation with the direct disposal tendency from residents in the riverbank area with low-managed of waste management.
As Jakarta obtain the immersed peak of leakage accumulation in February (Fig. 5) exceeding 2516.44 ton in one month to the overall waterway network in Jakarta, we investigated on the risk from higher rainfall rate (604.4 mm based on BMKG Kemayoran Meteorological Station ). Accordingly, we found the historical flooding events were peaked during the rainy seasons in Jakarta. Jakarta has occurred a long-time historical flood in February and spotted 1245 hotspots of flood from BPBD Jakarta . It is also implied that the understanding on the tendency of direct disposal was higher during the rainy season in the riverbank area which affected the significant runoff in the waterway causing the flood. Based on the research in the same location in Jakarta  in period 2015–2016, the tendency of direct disposal was up takingly high due to the higher flow and remove the waste from the residential area faster. In short, the higher rainfall became the complex issue where higher disposal and occurrence of the flash floods–leading to enormous number of plastic waste discarded in the river.
A significant drop identified in Jakarta calculations from February to March, where we found roughly 820.31 ton during March. We assumed a short-term solution during this period due to the overwhelming condition in the previous month. As taken respond of the catastrophic, the authority designated a major cleanup of the river area and disaster response. According to the Environment Agency in Jakarta, they conducted a special form of Pasukan Orange to prevent the flood occurring and do more rejuvenation of rivers in Jakarta . Therefore, we can conclude that the immediate act of plastic waste in short term can bring the significant impact and potentially lower the number of plastic waste in the river if a long-term act and predictive act across the systems can be enabled.
Similar to Jakarta area, the number of accumulations of plastic discarded to the river pathway are high during the rainy season period. As seen in Fig. 5, Bandung obtained the higher plastic waste in the river pathway during Ocotober to May. However, the extreme drop from May to June are alarmingly due to the sudden rainfall rate drop–which also a transition from rainy to dry season. This condition implies that the higher accumulation of plastic waste will happen in the rainy season. However, the higher disposal in the pathway can contribute to higheraccumulate accumulation of wastes in the dry season due to the lower runoff in the river. Therefore, we conclude that another cleanup activities needs to be implemented for the short-term solutions.