Commercial areas are generally fully paved and with more impervious land cover than residential areas. This paper demonstrates a wall-mounted rainwater harvesting system designed to deal with limited land space. An arrangement of three tanks in series was used on a commercial shop lot where flat roofs generate large amounts of runoff. The system is compact, and can be installed and fitted close to any wall, promoting the efficient use of space. Analytical procedures and computational fluid dynamic modelling were used to explore the system's potential. This rainwater harvesting system, with its three water storage tanks, works well, and is suitable for implementation and can be integrated into urban stormwater management.

INTRODUCTION

Rainwater harvesting is the interception of rainwater from roof systems so that it can be collected, stored, and possibly treated for reuse (Natibu & Mahoo 2000; Petrucci et al. 2012). The rainwater tank type and size are the major considerations for such a system (Worm & van Hattum 2006). Conventional rainwater tanks have substantial capacity, whether above or below ground (Morey et al. 2016), and occupy a lot of space that could be used for other purposes.

Wall-mounted tanks like those introduced here have smaller capacity than conventional ones and are installed on building walls (Tang & Mah 2015). Figure 1 presents examples of wall-mounted rainwater tanks on houses. Generally, sloping roofs are favored and house roofs have several sloping planes fragmented to varying surface areas. The tank shown transfers parts of the rainwater from the drainage downpipe. Too large a storage volume may result in a tank that it might not be feasible to suspend from a wall.
Figure 1

Wall-Mounted Rainwater Harvesting System on Residential Buildings (www.synergy-contract.com).

Figure 1

Wall-Mounted Rainwater Harvesting System on Residential Buildings (www.synergy-contract.com).

However, this paper deals with commercial buildings, which often have flat or gently-sloping roofs. They may use less water per capita than residential buildings, but harvesting rainwater could contribute to reducing rainwater volumes in urban drains, thus helping to mitigate flash flooding. Figure 2 shows typical shop lots in Malaysia, where rainwater is drained to both the front and back. The roof is a single plane with a large surface area. Placing rainwater tanks on the tightly-spaced lots requires a new approach and a series of wall-mounted tanks was thought suitable to cater for the greater amount of roof runoff. This paper outlines an investigation into the parameters for the wall-mounted tanks using analytical and computational fluid dynamics (CFD) methods.
Figure 2

Typical Shop Lots, (a) Front and (b) Back.

Figure 2

Typical Shop Lots, (a) Front and (b) Back.

RAINWATER HARVESTING SYSTEM

The basic components of a typical rainwater harvesting system comprise a catchment area, conveyance system and water storage devices (DID 2012). Technically, the catchment is the surface receiving the rainfall and supplying the system, and the conveyance (transfer) system comprises gutters, downpipes and piping (Dwivedi & Bhadauria 2009).

In order to check the effectiveness of a rainwater harvesting system, analytical procedures relate the flow rate of the associated piping system and tank storage volume over time (Figure 3). Rainwater enters a building drainage system at a given intensity, and its volume and velocity through the piping system can be simulated (Liao et al. 2015). When the rainwater reaches the tank, the latter's filling rate could be used to determine the optimum tank size. If filling is too quick the system has been under-designed, if it is too slow this indicates over-design.
Figure 3

Proposed Wall-Mounted Rainwater Harvesting System.

Figure 3

Proposed Wall-Mounted Rainwater Harvesting System.

CFD provides more detailed simulation than analytical procedures, and enables calculation of the velocity and pressure fields inside a control volume, over the x, y, and z dimensions at any specific time. The design flow rates determined in analytical procedures can be taken as the control volume, and plugged into CFD to visualize how the velocity and pressure fields act across the system, including all its bends, inlets, outlets, tight and broad spaces, etc. (Alnaimi et al. 2015; An et al. 2015).

ANALYTICAL ANALYSIS

A 15-minute, 10-year average recurrent interval design rainfall event is adopted here for design purposes. The storm duration considers the time taken for the rainwater to flow from the flat roof to an outlet; and 15 minutes are usually taken in normal practice. The design rainfall is taken from the official release of the Sarawak Department of Irrigation and Drainage (DID), in which an intensity of 180 mm/hr is derived based on the local weather variations.

When a shop lot with a roof catchment of 139.08 m2 is subjected to 180 mm/hr of rainfall for 15 minutes, the catchment could generate 6.3 m3 of runoff (as in Table 1). This scenario is taken as representing the worst-case. The amount of runoff is huge and therefore two outlets are required, by dividing them half each to outlets front and back of the shop lot. By placing wall-mounted tanks on either side of the shop lot's wall, 50% of the rainwater runoff can be harvested, and the volume concerned enables the design of water tanks. The other half is directed to urban drainage for disposal.

Table 1

Hydrologic analytical parameters

No. Design Parameters Data 
1. Roof area for a single shop lot 0.013908 ha/139.08 m2 
2. Design storm duration 15-minute, 10-year ARI 
3. Rainfall intensity 180 mm/hr 
4. Volume of roof runoff generated By using Rational Method,
Q = cIA/360 (c = 1 for 100% imperviousness of roof)
=(1 × 118 mm/hr × 139.08 m2)/360
= 0.006954 m3/s 
V = Q × duration of storm
= 0.006954 m3/s × (15 × 60) s
= 6.3 m3 (half to front, half to back) 
5. Roof runoff using Rational Method Qin = 0.003475 m3/s (to rainwater tanks) 
6. Total volume of three rainwater tanks 3(1.5 × 1.0 × 0.7 m) = 3.15 m3 
7.  Velocity at Outfall (m/s) Runoff at Outfall (m3/s) 
 Using 60 mm pipe v1 = 6.9 Qout1 = 0.012000 
 Using 90 mm pipe v2 = 1.8 Qout2 = 0.000131 
 Using 120 mm pipe v3 = 2.0 Qout3 = 0.000205 
No. Design Parameters Data 
1. Roof area for a single shop lot 0.013908 ha/139.08 m2 
2. Design storm duration 15-minute, 10-year ARI 
3. Rainfall intensity 180 mm/hr 
4. Volume of roof runoff generated By using Rational Method,
Q = cIA/360 (c = 1 for 100% imperviousness of roof)
=(1 × 118 mm/hr × 139.08 m2)/360
= 0.006954 m3/s 
V = Q × duration of storm
= 0.006954 m3/s × (15 × 60) s
= 6.3 m3 (half to front, half to back) 
5. Roof runoff using Rational Method Qin = 0.003475 m3/s (to rainwater tanks) 
6. Total volume of three rainwater tanks 3(1.5 × 1.0 × 0.7 m) = 3.15 m3 
7.  Velocity at Outfall (m/s) Runoff at Outfall (m3/s) 
 Using 60 mm pipe v1 = 6.9 Qout1 = 0.012000 
 Using 90 mm pipe v2 = 1.8 Qout2 = 0.000131 
 Using 120 mm pipe v3 = 2.0 Qout3 = 0.000205 

The harvested rainwater could be reused, for which a pipe would connect the tank to the intended appliances, or released to the environment via a downpipe. Neither connection is included in the analysis, which deals only the critical mass flow into and out of the system. It is also limited in its detail, such as energy losses in pipe connections.

CFD SIMULATION

There are three inlet designs. Design 1 has the rainwater runoff distributed through three arms that is similar to a T intersection, therefore such a design is termed T-Inlet from this point onwards. Design 2 has a sharp 90 ° bend that resembling a L shape, therefore it is termed L-Inlet. Design 3 has an inclined distribution inlet pipe with a 10° slope, which is termed I-Inlet. The 10° slope is chosen so that to have a consistent length from inlet opening to tanks for all three designs.

Generally, T- and L-Inlets are designed with one tank which connects directly to the inlet opening – the middle tank for T-Inlet and the left tank for L-Inlet (Figure 4(a) and 4(b)). In such a design, the named tanks will receive the greatest volume of water by gravity. Meanwhile, the I-Inlet has no direct connection to the inlet opening (Figure 4(c)).
Figure 4

Types of Inlet Design, (a) T-Inlet, (b) L-Inlet and (c) I-Inlet.

Figure 4

Types of Inlet Design, (a) T-Inlet, (b) L-Inlet and (c) I-Inlet.

Expansion and contraction occur commonly in pipelines with connections and appliances, and dimensional changes in flow systems have profound implications on flow patterns. The energy used to move water through pipelines can neither be created nor destroyed, but must be conserved locally. Generally, water tends to flow faster in smaller diameter pipes and the resultant pressure is lower; once it passes to larger diameter pipes, the velocity is lower and the pressure is higher, to conserve energy. In pipeline-tank networks, dimensional changes occur when a pipeline is connected to a tank to accommodate inflow or a tank to a pipe for outflow (Liu et al. 2016).

Three modelling scenarios with common tank sizes are represented in CFD. Scenario 1 involves 60 mm pipe throughout the system, while scenarios 2 and 3 use 90 and 120 mm pipes, respectively (Figure 5). There are thus nine (9) sub-models.
Figure 5

Simulated Streamlines in Rainwater Harvesting Tanks.

Figure 5

Simulated Streamlines in Rainwater Harvesting Tanks.

In an enlarging joint, the water velocity changes from relatively high in the pipe to low when entering a comparatively large tank, and the velocity reduction causes a sharp rise in pressure. In a contracting joint, the opposite happens. Generally, flow equilibrium tends to occur in the lower pressure and high velocity zone first – i.e., at the inlets and outlets, and within the pipes – but not in the tanks. This suggests that the tanks' velocity/pressure could be used to determine the optimum design.

Different inlet designs distribute water differently, creating various flow patterns in the tanks. Representing the flow pattern in streamlines shows how the water travels. All scenarios portray low velocity and high pressure in the tanks (Figure 6). The streamlines appear to be less intense when flow is fast (Figure 5(g)), but chaotic when slow (Figure 5(a)5(c)). Further CFD analysis enables determination of the design offering the most balanced flow in the tanks.
Figure 6

Simulated Pressures in Rainwater Harvesting Tanks.

Figure 6

Simulated Pressures in Rainwater Harvesting Tanks.

For Scenario 1, the CFD estimate of the maximum velocity at the outfall was in the range 4.4 to 4.7 m/s. The analytical procedure yielded an estimate of 6.9 m/s. There were intense streamlines in all tanks indicating slow flows. For T- and L-Inlets, the tank with direct connection to the inlet opening shows the highest pressure among the three in each design (Figure 6), indicating that the direct connection has forced the water to drain faster than the other tanks, increasing the risk of choking there.

For Scenario 2, the analytical procedure and CFD yield maximum outfall velocity estimates between 1.8 and 2.0 m/s. The streamlines exhibit extremes, however, in which one tank always has low intensity streamlines while the other two are chaotic. This is an explicit demonstration of unbalanced flow/pressure in the tanks.

The analytical procedure for Scenario 3 yielded a maximum outfall velocity estimate of 2 m/s. CFD, on the other hand, gave about 1.2 to 1.4 m/s for all designs. T-Inlet has low intensity streamlines, indicating fast flow/low pressure; with both L- and I-Inlet, however, one tank is significantly more chaotic than the others, the I-Inlet design having the most nearly balanced velocities between the three tanks (Table 2).

Table 2

Summary of modelling scenarios

Design Pipe Size (mm) Maximum Velocity at Tank Outlet (m/s)
 
Maximum Velocity at Outfall (m/s)
 
Left Tank Middle Tank Right Tank Indicator Analytical CFD 
Design 1-1 60 0.256 1.163 0.974 Unbalanced 6.9 4.717 
Design 2-1 1.076 0.254 0.983 Unbalanced 4.717 
Design 3-1 0.492 0.619 1.208 Unbalanced 4.420 
Design 1-2 90 0.152 0.497 0.390 Unbalanced 1.8 2.026 
Design 2-2 0.507 0.152 0.397 Unbalanced 2.029 
Design 3-2 0.211 0.320 0.506 Unbalanced 1.998 
Design 1-3 120 0.088 0.279 0.210 Unbalanced 2.0 1.241 
Design 2-3 0.277 0.080 0.224 Unbalanced 1.237 
Design 3-3 0.118 0.182 0.296 Balanced 1.449 
Design Pipe Size (mm) Maximum Velocity at Tank Outlet (m/s)
 
Maximum Velocity at Outfall (m/s)
 
Left Tank Middle Tank Right Tank Indicator Analytical CFD 
Design 1-1 60 0.256 1.163 0.974 Unbalanced 6.9 4.717 
Design 2-1 1.076 0.254 0.983 Unbalanced 4.717 
Design 3-1 0.492 0.619 1.208 Unbalanced 4.420 
Design 1-2 90 0.152 0.497 0.390 Unbalanced 1.8 2.026 
Design 2-2 0.507 0.152 0.397 Unbalanced 2.029 
Design 3-2 0.211 0.320 0.506 Unbalanced 1.998 
Design 1-3 120 0.088 0.279 0.210 Unbalanced 2.0 1.241 
Design 2-3 0.277 0.080 0.224 Unbalanced 1.237 
Design 3-3 0.118 0.182 0.296 Balanced 1.449 

CONCLUSIONS

In this project, a rainwater harvesting system with wall-mounted tanks was demonstrated successfully and tested by computer modeling. Three scenarios are described, and the results presented graphically in the forms of streamline and pressure zonation, supported by the velocities generated in the tanks. The modeling shows the performance of the flow entering the system under various inlet designs, in relation to three pipe sizes.

Of the three inlet designs, the I-Inlet has the most consistent flow patterns/streamlines in the tanks for all three pipe sizes. The T- and L-Inlets designs have faster flows with increasing pipe size. Despite that, they exhibit different streamlines/pressure zones corresponding to the three pipe sizes – the 60 mm pipe produces the slowest flow in the tanks.

The I-Inlet rainwater harvesting system should have the most balanced velocity characteristics. Design 3-3 (I-Inlet with 120 mm pipe) is considered the best for rainwater harvesting in the limited spaces of a close-knit built environment.

ACKNOWLEDGEMENT

The first author received financial support from the Universiti Malaysia Sarawak's Special Grant Scheme F02/SpGS/1405/16/6. The second (project leader) and third authors (collaborator) would like to express gratitude to the same grant.

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