Determining demand for water, water supply and drainage balance to wastewater reuse for urbans in Vietnam

. Wastewater reuse is very important in ensuring a stable water supply for the socio-economic development of cities in the future. That is even more meaningful for areas affected by climate change erratic, hot, arid, scarce and polluted due to different causes. Specifically, many regions and urban areas in Vietnam have not been proactive in water resources upstream; runoff through agricultural, industrial and urban areas contaminated by farming, industrial waste, wastewater and municipal solid waste. Based on published studies on the role and situation of wastewater reuse in urbans, as well as on legal documents Vietnam's current management related to wastewater drainage and reuse, the article presents how to calculate and determine the water demand in urban areas for calculating capacity of water supply plants; to set up the balance diagram of water supply and drainage for all types of urban areas (from special to grade V urbans) and the balance diagram of water supply and drainage in the works. The research results will be considered as a scientific basis for state management agencies as well as local authorities to appropriately and effectively use in formulating strategic orientations and objectives for urban water supply and drainage management in Vietnam urban areas.


Introduction
The situation of urban domestic wastewater treatment in Vietnam: As of November 2022, the whole country has about 71 concentrated urban domestic wastewater treatment stations with a total design capacity of approximately 1,380,000 m 3 /day and operating capacity accounting for about 15% of the domestic waste water total volume was generated [3].The role of urban wastewater reuse in Vietnam: Water reuse brings economic benefits from saving the volume of clean water and raw water extracted from natural sources by the volume of reused wastewater and environmental benefits from reducing a similar volume of wastewater discharge.
Situation of wastewater reuse in Vietnam: In Vietnam, at present, the statistics, inventory and management of wastewater reuse have not been officially implemented in any industry, field or locality in our country.The management of wastewater reuse has not been clearly and specifically assigned or decentralized.At the macro level, the issue of reuse of wastewater after treatment has not been given due attention and attention; In the master plan on water resources, integrated master plan on river basin, as well as planning on water exploitation and use for economic sectors, almost no mention is made of the problem of reuse of wastewater after treatment.At the micro level, the issue of wastewater reuse for investment projects has not been recognized or considered right from the planning, design or appraisal stages.Vietnam has also had applications to reuse wastewater in agriculture and aquaculture.That has brought many economic benefits from increased crop yields and increased fish production.In the high mountains, islands and coastal areas where fresh water is scarce, people have known how to collect, store and treat rainwater for use for drinking and living.Production, trade and service facilities in the provinces of Binh Thuan, Da Nang, Khanh Hoa,... has taken advantage of treated wastewater (up to standard) to irrigate plants.In many applications for permits to discharge wastewater into water sources of paper mills, sugar production, thermal power plants, mining and mineral processing plants, etc., which were sent to the Ministry of Natural Resources and Environment, there was mention of the reuse of wastewater after treatment for activities such as washing raw materials, irrigating roads, cleaning equipment, watering plants in the factory premises... [6].Nevertheless, in Vietnam, there are currently no specific projects on wastewater treatment for recycling that used for non-drinking water supply purposes in urban areas.There are only wastewater treatment solutions to replenish urban lakes and ponds with the aim of creating environmental landscapes and regulating microclimates, water supply in construction and irrigation in agriculture...In the context of The country's socio-economic situation is developing strongly, which requires an increased demand for water.As climate change and extreme weather are adversely affecting natural water supplies, the reuse of wastewater can provide many economic, social and environmental benefits as well as offset some of it for water supply problems.
Current legal documents that Vietnam is applying regarding reuse of urban domestic wastewater: Wastewater reuse is one of the contents of Goal 6 -ensuring adequate and sustainable management of water resources and sanitation for all (Sustainable Development Goals -SDG6) under 17 Sustainable Development Goals proposed by the United Nations and committed by member states until 2030 [5].For Vietnam, the reuse of wastewater in a planned, strategic, reasonable, clear and effective manner will make an important contribution in stabilizing businesses' economic development and in the future planning for management and protection of natural resources and environment for managers; contributing to the making of the national sustainable development goals in order to apply Decision No. 681/QD-TTg issued on June 4, 2019 on the Roadmap for the implementation of Vietnam's sustainable development goals by 2030 [8] and Resolution No. 136/NQ-CP dated September 25, 2020 on sustainable development that the Government has set [7].In Decree 80/2014/ND-CP [11], Article 24 stipulating the management and use of treated wastewater must comply with TCVN and QCVN regulations for the use of water for different purposes.affect the health of the people and ensure the safety of the environment.In case of using treated wastewater, that wastewater must be distributed to the point of consumption according to a separate system, ensuring that it does not penetrate and affect the clean water supply system in the same location or area.The Ministry of Natural Resources and Environment shall assume the prime responsibility for, and coordinate with concerned ministries and branches in, promulgating TCVN and QCVN on the use of treated wastewater.In Decree 38/2015/ND-CP [10], Article 36 stipulating the general principles of waste management also clearly states that wastewater reduction and reuse is encouraged: Wastewater must be managed through mitigation activities., reuse, collect and treat up to environmental standards; encourage activities to reduce and reuse wastewater.Article 40: Wastewater after treatment must be collected for reuse purposes.The reuse of treated wastewater must comply with specific regulations for each use purpose.Circular 04/2015/TT-BXD of the Ministry of Construction, Article 1 of regulations on decentralized wastewater treatment management, mentioned the technologies applied to implement decentralized wastewater treatment as "constructed westland" and "regulating pond" [2].In a general sense, if these technologies are applied, it means that wastewater is collected and can be reused.According to Decision No. 589/QD-TTg, dated April 6, 2016, approving adjustment of development orientations for urban drainage and industrial zones in Vietnam to 2025 and a vision to 2050 [9 ]: to 2025 on both Water is expected to have 50% of the total wastewater volume of grade II cities and above and 20% for urban centers of grade V and above to be collected and treated to meet technical standards and regulations before being discharged into the environment.Expand the service scope of urban drainage system to reach over 80% of service coverage on average, 80% of wastewater from craft villages is collected and treated up to standards and technical regulations before When discharged into urban drainage systems or into the environment, 20-30% of wastewater after treatment meets technical standards and regulations can be reused for watering plants, washing urban roads and other needs.Since 2015, the Technical Committee ISO/TC 282 Water reuse of the International Organization for Standardization (ISO) has compiled and published the first international standards on water reuse by ISO.In order to timely integrate with these international standards in the spirit of "standardization is one step ahead", in 2017, 2018 and 2020 Vietnam's national standards bodies have accepted the first Vietnamese standards on water reuse and has been announced by the Ministry of Science and Technology.Among them, the group of standards on water reuse in urban areas includes: the set of TCVN 12525:2018 (ISO 20760:2018) (2 parts) on Guidelines for centralized water reuse systems [12] and TCVN 12526:2018 (ISO 20761:2018) on Guidelines for safety assessment of water reuse -Parameters and assessment methods [13], provides parameters and methods to safely assess the reuse of non-potable water in urbans that are intended to assist water engineers, regulators, decision makers and stakeholders in determining the safety of renewable water for end uses.
Urban classification: In Vietnam, urban areas are classified into 6 categories, which are grade I, grade II, grade III, grade IV, grade V and special grade.As of December 2022, Vietnam has 888 urban areas with two cities classified by the Government as special urban areas: the capital Hanoi and Ho Chi Minh City; 22 grade I urbans (including 3 cities directly under the central government and 19 cities under the province); 33 grade-II urbans (all of which are provincial cities); 47 grade III urbans (including 29 cities and 18 towns); 94 grade IV urbans; 690 grade V urbans.[4] 2 Methodologics and data used for the study • Theoretical research method on how to determine water demand in urban areas according to classical formulas combined with application to the actual conditions of Vietnam

Data on urban water supply services used in the study
It is possible to identify the needs of water supply services in urban areas according to the regulations on standards of domestic and industrial water supply, physical training and sports facilities, irrigation of green parks, golf courses.Demand for urban water supply services is shown in Tables 1 and 2.
C Industrial and service water in urban areas q o(cn) , m 3 /ha.day;Q 3 , m 3 /day F-area, ha E Flush water wash toilet Q 4 = k 4 .qo .N q o , l/cap/day; Q 4 , m 3 /day k 4 = 30% [15] F Leaking water in the water supply system G Water for water treatment plant specific requirements Water for golf courses and sports facilities F Leaking water in the water supply system G Water for water treatment plant specific requirements Note: (*) -Order C (industrial and service water in urban areas) in Table 2 proposed by the authors by 2030 and vision to 2050.

Urban water supply capacity
From the calculated data listed in Tables 1 and 2, the urban water supply capacity can be determined.
• For urban centers of grade III or higher: In which: k 1 , k 2 , k 5 -coefficients taken according to the instructions in the reference column of table 1.
• For urban centers of grades IV and V: In which: k 1 , k 2 , k 5 -coefficients taken according to the instructions in the reference column of table 2.

Water plant capacity (in case wastewater is not reused)
From the calculated data listed in Tables 1 and 2, the capacity of the water supply plant can be determined: • For urban centers of grade III or higher: In which: k 1 , k 2 , k 4 , k 5 , k 6 -coefficients taken according to the instructions in the reference column of table 1.

Demand for non-drinking water purposes in urban areas (taken from reused wastewater)
The demand for non-drinking water purposes in urban areas addressed in this study includes: • Demand for using public water for watering plants, washing roads, fighting fire works; • Demand for domestic water for house cleaning, flushing urinals and • Demand for water for industrial services in urban areas such as commercial water, water used in production processes in buildings, offices, etc. From the data listed in Tables 1 and 2, a formula can be built to calculate the demand for for non-drinking water purposes in the urban area that taken from reused wastewater as follows: • For urban centers of grade III or higher: Q cnđt(III,tsd) = Q 1 +Q 2 +Q 4(III) +Q 5 = k 1 q o N+k 2 q o N+k 4 q o N+k 5 (k 1+ k 2 +k 4 )q o N = = [k 1 +k 2 +k 4 +k 5 ( k 1 +k 2 +k 4 )]q o N = 0,6.qo .N (5) In which: k 1 , k 2 , k 4 , k 5 -coefficients taken according to the instructions in the reference column of table 1.

Water plant capacity (in case of wastewater reuse)
From the calculated data listed in Tables 1 and 2, the capacity of the water plant can be determined when taking into account the reuse of wastewater for non-drinking water purposes in the urban area as follows: • For urban centers of grade III or higher + Capacity of water supply plant from natural water source: • For urban centers of grades IV and V + Capacity of water supply plant from natural water source: + Capacity of water supply plant from reused wastewater: ≈ 0,589.qo .N (10) 4 Solution for water supply and drainage balance diagram

Overall diagram of urban water supply and drainage balance
From the calculation data of water supply services in Table 1 -for urbans of grade III and above and table 2 -for urbans of classes IV and V, it is possible to calculate the capacity of urban water supply and water plants in different areas. in the case of with and without reuse of wastewater, and at the same time calculate the discharge and discharge volume to the receiving source.From that, an overall diagram of urban water supply and drainage balance can be built as shown in Figure 1.
The assumptions for the overall diagram of urban water supply and drainage balance is as follows: • Assumption 1: There exist two parallel networks in urban areas, one is a clean water supply network and the other is a reused water supply network.Water from the clean water supply network and urban reuse water supply network is directed to objects with different uses.
• Assumption 2: The water supplied after use becomes waste water, which is led out of the works according to the drainage system and then brought to the centralized urban wastewater treatment station.The treated wastewater is disinfected and a part discharged into the receiving source, the rest is further treated with advanced treatment for reuse for non-drink water purposes and with the quality of treated wastewater meeting the requirements for reuse waste water.

o Balance diagram of water supply and drainage in the works
Water demand in the works includes: -Water for bathing, cleaning, flushing toilets and washing floors (usually used from restrooms); -Water for food preparation, cooking rice and drinking water (usually used from the kitchen); -And water for services such as watering trees around the works, washing the yard, internal roads and fire fighting of the works (depending on the location of water use and other specific characteristics, water can be used from the external or internal works water supply system) On the basis of the water demand in the works, the principle of balancing water supply and drainage in the works is established as shown in Figure 2.
According to the diagram of Figure 2, the design requirements for parts of the works water supply and drainage system to ensure the reuse of wastewater are:  b) Water meter cluster: For the works water supply system with reuse wastewater, the number of water meters must be at least 02: (i) Used to measure the flow of drinking water supply and (ii) Used to Measure the flow of non-drinking water supply.c) Piping system, shut-off valves, water-using equipment in the works: Calculation and use of pipes, valves and water-using equipment as for the water supply system.However, in the case of reuse wastewater, the piping system, shut-off valves and water-using equipment are selected and used for both drinking and non-drinking water supply systems.d) Water storage tanks and flow conditioners They are underground water tanks and water tanks (roof water tanks and water tanks serving water supply areas if the high-rise building has many water supply zones).
• Underground water tanks: There are at least 02 underground water storage tanks in the works with the suggested volume calculated as follows: + For water storage and conditioner compartment with non-drinking purposes: W tsd = (2,5*3,6*3) + 50%.W b = 27 + 0,5.W b (m 3 ) (11) + For water storage and conditioner compartment with drinking purposes: W au = 0,5.W b = 0,5.Q b .t(m 3 ) (12) In which: 2.5l/s -fire fighting water flow; 3.6 -unit conversion factor l/s to m 3 /h; 3fire fighting time in 3 hours; W b -the amount of conditioners water is determined depending on the working mode of the urban network (conditioner) and the water pumping mode from the underground tank to the water tanks of the building: W b = Q b *t (Q b -the pump capacity m 3 /h; t-water pumping time, h); • Water tanks: The water tanks are designed to store and conditioner the amount of pumped water and the amount of water consumed in the building, calculated according to the conventional method.The water tanks are designed with 02 compartments, in which the reuse wastewater tank has a capacity equal to the capacity of the drinking water tank plus the amount of fire-fighting water in 10 minutes.

Discussing the research results
Urban water supply is an indispensable problem, in fact there are countries, because of the scarcity of fresh water supplies, have to buy raw water from other countries (such as Singapore having to buy raw water from Indonesia) to treat the water supply for domestic and industry.Supply water after being used will become wastewater and is considered an available water source in the urban.If this type of water is not treated and discharged, it will pollute the receiving source, on the contrary, if it is treated effectively and thoroughly, but only for discharge, it is very wasteful.As such, wastewater is a recyclable and reusable resource in the urban for a variety of purposes and provides economic, social and environmental benefits.Based on the research results, the reuse of wastewater in urban areas in Vietnam will bring specific benefits, including: • Savings in natural water supply down to 44.1% for grade III urbans and higher; ~46.6% for grade IV and V urbans.That is especially beneficial for urbans lack of water sources for urban development; • The amount of wastewater discharge can be reduced by up to 41.7% for grade III urbans and higher; ~37.9% for grade IV and V urbans.Thus, the disturbance, dilution as well as the self-cleaning ability of the receiving source will be better and thereby the environmental protection work will be simpler and more effective.All the situations that occur when there is reuse of non-drinking water in urbans will create favorable conditions for environmental protection and sustainable urban development.

Conclusions
The article has studied the calculation and establishment of the overall diagrams of water supply and drainage balance in the urban and the works.In there: • The overall diagram of urban water supply and drainage balance has clarified the relationship between water supply from natural sources and sources of wastewater reuse, drainage and discharge.Waste water is treated, a part (about 0.508q o N for urban areas of grade III and higher; 0.515q o N for urbans of grades IV and V) is disinfected and discharged into the receiving source, the other part (about 0.644q o N for urban centers of grade III and higher and 0.589q o N for urban centers of grades IV and V) is treated with advanced treatment to reuse for non-drinking purposes.• Balance diagram of water supply and drainage works with 2 water supply systems: (i) For drinking from natural water sources, in which the capacity of underground storage tanks and roof tanks is proposed to be calculated with day and night conditioning coefficients; (ii) Non-drinking (from reused wastewater to flush urinals, house cleaning, watering plants, washing internal gardens, creating fountains landscape, fighting fires, etc.) Calculation analysis with day and night air conditioning coefficient, supplemented with 10 minutes of fire fighting for the roof

2. 1
Methodologics• Methods of collecting documents and data on: (i) Wastewater treatment in urban areas of Vietnam; (ii) Situation of wastewater reuse in Vietnam• Methods of analysis, evaluation and synthesis of data: Synthesize and analyze to assess the current status of wastewater treatment and reuse, analyze and select limited scopes in regulations on water supply and drainage in Vietnam current legal documents.

Figure 1 .
Figure 1.Overall diagram of urban water supply and drainage balance Note: q o -standard of water supply, liters/cap/day; N -calculated population, people; NMN-water treatment and supply plant from resource; XLNT-Urban wastewater treatment station; TXLNC-Advanced wastewater treatment station for reuse; Wastewater reuse network; Water supply network for drinking and domestic use from natural water resource

Figure 2 .
Figure 2. Diagram of water supply and drainage system in the works with wastewater reuse a) Water pipe to the house: For the works water supply system with reuse wastewater, the incoming water pipeline must be designed with 02 pipes: (i) Used for drinking water and bathing and (ii) For non-drinking purposes.Pipe sizes are taken equal, calculated on the basis of capacity ½ maximum daily flow.b)Water meter cluster: For the works water supply system with reuse wastewater, the number of water meters must be at least 02: (i) Used to measure the flow of drinking water supply and (ii) Used to Measure the flow of non-drinking water supply.c) Piping system, shut-off valves, water-using equipment in the works: Calculation and use of pipes, valves and water-using equipment as for the water supply system.However, in the case of reuse wastewater, the piping system, shut-off valves and water-using equipment are selected and used for both drinking and non-drinking water supply systems.d) Water storage tanks and flow conditioners They are underground water tanks and water tanks (roof water tanks and water tanks serving water supply areas if the high-rise building has many water supply zones).•Underground water tanks: There are at least 02 underground water storage tanks in the works with the suggested volume calculated as follows: + For water storage and conditioner compartment with non-drinking purposes: W tsd = (2,5*3,6*3) + 50%.W b = 27 + 0,5.W b (m 3 )

Table 1 .
Demand for water supply services in urban areas of III grade or higher

Table 2 .
Demand for water supply services in urban areas of IV grade and V grade