Utilization of rainwater harvesting to conserve water in buildings in Semarang City

Rainwater harvesting makes a major contribution to sustainable development. The utilization of rainwater for building operations is in line with sustainable development goals for 2030. This research aims to provide an overview of rainwater which can be used as an alternative source of raw water in buildings in Semarang City. This research was carried out purposively on buildings in the city of Semarang that had received function-worthiness certificates. The research results show that the largest rainwater harvesting dimension is the Ciputra building of 372 m3 with rainwater potential that can be utilized in the form of runoff of 0.758 m3/second or 65491.2 m3/day, while the smallest dimension is the Pusatand building of 55 m3 with rainwater potential which can be utilized in the form of runoff of 0.112 m3/second or 9679.8 m3/day. Municipal waterworks (PDAM) tariff costs have decreased after carrying out rainwater harvesting. The largest percentage decline was in the Ciputra building, namely 65.72%, while the smallest was in The Pinnacle at 5.16%. The size of the Municipal waterworks (PDAM) water tariff is determined by the ground floor area of the building since the ground floor area contributes to the dimensions of rainwater harvesting.


Introduction
One of the 17 Sustainable Development Goals for 2030 is rainwater harvesting (RWH).These goals include access to clean water (Goal 6), affordable and clean energy (Goal 7), sustainable cities and communities (Goal 11), and action to avoid the impacts of climate change (Goal 13).RWH is a promising technology to help achieve these goals.Rainwater harvesting (RWH) can provide water for on-site consumption as a decentralized primary or supplementary source.Rainwater can be used for both drinking and non-drinking purposes, which means the conservation of raw and treated water as well as the saving of resources used in water treatment and distribution [1] [2].
The RWH system can help the sustainability of cities and villages.Aside from the more obvious benefits (such as the ability to supply water in a decentralized manner and increase local water security), RWH can also have a positive impact on the environment.These benefits include saving drinking water, reducing water use for building operations like watering plants and flushing, and reducing runoff in urban areas [3].One low-impact development solution used to restore the natural water cycle in cities is to use rainwater harvesting [4].To carry out rainwater harvesting, several variables are needed, such as local rainfall data, roof area, type of water catchment surface, rainwater needs, and drinking water needs [5].The application of RWH for sustainable water management entails numerous factors, including decentralization of public water delivery systems and flood prevention [6].RWH is one method for diversifying water sources and increasing water security [7].RWH has the potential to lessen the environmental effects of buildings and cities, making them more sustainable [8].
Saving water resources through rainwater harvesting is governed by Semarang Mayor Regulation No. 24 of 2019, which governs green building.Section 6 of Article 17 states: rainwater harvesting as referred to in paragraph (1) letter c, must be collected and stored in a raw water tank to be processed and used as primary water.The primary water in question is water that can be used for daily building operations.Since the enactment of the Semarang Mayor Regulation, all buildings in Semarang City must utilize rainwater as an alternative source of raw water.The method of water conservation is to collect most of the rainwater from the roof into a raw water tank, and then use it for various purposes.
If not processed, it can be used for watering plants, flushing, bathing livestock, etc.If processed properly it can be used for household purposes and even for drinking water.
Based on the background above, it is important to carry out RWH as a water conservation effort in buildings in Semarang City.

Methodology 2.1. Location and research sample
The research sample consisted of 10 buildings in Semarang City with a minimum size of 5,000 m2 including the basement.The sample was carried out purposively, namely by deliberately selecting buildings that had been assessed by the Professional Team of Experts (TPA) and had received a Function-Worthiness Certificate (Sertifikat Laik Fungsi or "SLF").

Data analysis
The data used to analyze rainwater harvesting to reduce runoff in buildings in Semarang City are the intensity of rainwater, the area of the ground floor of hotel and apartment buildings, and runoff discharge.

Calculation Method
In analyzing runoff discharge and rainwater discharge, several stages are required in the analysis, namely: Where : e. Rainwater harvesting dimensions The dimensions of rainwater harvesting are calculated using the formula: RWH dimensions = 0.025 m x ground floor area (m 2 ) ( f. Building water demands Calculation of water requirements is calculated based on data on the effective floor area.The effective floor area is between 55 and 80 percent of the total area of the building [10].In this research, the effective floor area is taken as 60% with a density level of 5 m 2 /person, then after the effective floor area is determined the population size in each building can be calculated.The population size in a building can be calculated using the following equation: Where: No = occupant count (person) Ef = effective floor area (%) A = total building area (m 2 ) Od = occupancy density level (m 2 /person) Once the number of residents is known, water requirements can be calculated using equation (7).The standard water usage used for buildings is 250 liters/person/day [11].Qd = number of residents x standard water usage (7) Where : Qd = water requirement (m 3 /day)

Analysis of Average Maximum Rain Intensity Using the Mononobe Method
Rainfall intensity at the research location was calculated using the Mononobe method.The data needed to calculate this rainfall intensity is the average rainfall data per second, which is the amount of rain in a particular month for a particular year, also known as the average daily maximum rainfall (R24).This intensity value can be obtained by calculating factor parameters such as maximum daily rainfall in 24 hours (R24) and rainfall duration (t).Calculation of rainfall intensity using the Mononobe Method is shown below: The results of calculating the average hourly rainfall using the Mononobe method according to equation ( 2) are as follows:

Runoff Discharge Analysis
Based on the data obtained, the buildings are all located in the research area with flat topographic conditions, clay soil conditions, and vegetation without plants so that based on equation (3), the runoff coefficient C = Ct + Cs + Cv is 0.03 + 0.16 + 0.28.= 0.47.The C, I, and A values that have been obtained are then used to calculate the runoff discharge in buildings in Semarang City using equation (4).
Runoff discharge analysis was carried out by processing rain data from 2020-2022.It is based on three years of continuous rainfall data, which meets the results of frequency analysis and can be used for minimal analysis.Apart from rainfall data, topographic data is needed to determine the area of the area studied, as well as the slope length (L) and slope (S), so it was found that all buildings in the research area are on a slope length of 0.245 km and a slope of 0.5 %.This data is used to calculate the concentration-time (tc) according to the following equation ( 1 Once the coefficient value, rain intensity, and ground floor area of the building are known, the runoff discharge can be calculated.The runoff value can be calculated using equation ( 4).The largest runoff discharge is the Ciputra building, namely 0.758 m 3 /second, this happens because this building has a larger ground floor area (A) in the Ciputra building compared to other buildings, while the smallest runoff discharge is the Suite Hotel building, namely 0.051 m 3 /second because this building has a smaller ground floor area (A) compared to other buildings.
Based on Table 3, it can be concluded that the existing conditions in each building before rainwater harvesting was implemented produced different run-off discharges.These run-off discharges have great potential if used as an alternative source of raw water in buildings [12].

Dimensional Analysis of Rainwater Harvesting
The dimensions of rainwater harvesting are calculated using equation (5).The dimensions of rainwater harvesting are calculated after the ground floor area of the building is known, then multiplied by a factor of 0.025 m.The dimensions of rainwater harvesting are the same as the dimensions of raw water tanks which are regulated in Semarang Mayor Regulation Number 24 of 2019 concerning green buildings.Details of the dimensions of rainwater harvesting for each building can be seen in Table 4:

Water Demands Analysis
Analysis of water needs can be calculated using equations ( 6) and (7).Equation ( 6) is used to determine the number of building occupants based on the effective floor area.After the effective floor area is determined, the next step is to calculate the clean water needs of each building.Water requirements in buildings can be seen in Table 5 below:

Conclusion
Based on the research results, the following conclusions were obtained: The dimensions of rainwater harvesting depend on the ground floor area of the hotel and apartment building.The largest dimension is the Ciputra building of 372 m 3 with rainwater potential that can be utilized in the form of runoff of 0.758 m 3 /second or 65491.2m 3 /day, while the smallest dimension is the Centraland building of 55 m 3 with potential rainwater that can be utilized in the form of runoff amounting to 0.112 m 3 /second or 9679.8 m 3 /day.
Municipal waterworks (PDAM) tariff costs have decreased after carrying out rainwater harvesting.The largest percentage decline was in the Ciputra building, namely 65.72%, while the smallest was in The Pinnacle at 5.16%.The size of the PDAM water tariff depends on the ground floor area of the building because the ground floor area will contribute to the dimensions of rainwater harvesting.

Figure 1 .
Figure 1.Research Location Map[9] discharge (m 3 /sec) C = Watering coefficient I = Rainfall intensity (mm/hour) A = Building ground floor area (km 2 ) Using a three-year repeating cycle, the highest 24-hour rainfall was 82.76 mm.This value is then used to calculate rainfall intensity using the Mononobe method.Results for the previous three-year period using equation (2) are as follows:

Table 2 .
Recapitulation of Rainfall Intensity in 1-24 hours for the 2020-2022 period

Table 3 .
Runoff discharge for hotels and apartments in Semarang City

Table 4 .
Dimensions of rainwater harvesting in hotels and apartments in Semarang City

Table 5 .
Water requirements for hotels and apartments in Semarang CityBased on Table5above, the cost of using municipal waterworks (PDAM) water can be calculated.Based on Semarang Mayor Regulation Number 31 of 2019 concerning Drinking Water Tariffs at the Tirta Modal Regional Public Company for Drinking Water, Semarang City, buildings are included in the Commercial VI classification with detailed tariffs above 31 m 3 of Rp. 14000, so details of municipal waterworks (PDAM) water costs before and after RWH can be seen in Table6.

Table 6 .
Municipal waterworks (PDAM) water costs before and after RWH