9.2 Design Loading Rates (DLR), Reserve Areas & Hydraulic Sizing

Source boundary: Area = daily design flow / design loading rate is only as sound as its approved inputs. Daily flow, loading rate, geometry, dosing, and reserve area must be justified by the site assessment, AS/NZS 1547 method, and authority requirements.

Design Loading Rates (DLR), Reserve Areas & Hydraulic Sizing

The fundamental objective of on-site wastewater engineering is to achieve a permanent state of hydraulic and biological equilibrium between the daily wastewater volume produced by the building occupants and the assimilative capacity of the receiving soil. If the hydraulic loading exceeds the soil's natural permeability, the pore spaces remain continuously saturated with water. This cuts off atmospheric oxygen transfer, promotes anaerobic bacterial slime growth, and leads inexorably to surface ponding, raw sewage breakouts, and catastrophic environmental failure.

To ensure consistent, science-based sizing across New Zealand, AS/NZS 1547:2012 provides standardized tables for Design Loading Rates (DLR) for trenches and beds, and Design Irrigation Rates (DIR) for subsurface drip irrigation. Certifying drainlayers must be fully conversant with these loading tables, the underlying mathematical formulas, and the statutory mandate to secure an uncompromised 100% reserve area on every site plan.

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1. Soil Categories and Loading Rates under AS/NZS 1547

Soil is classified into six distinct categories based on soil texture, structure, and saturated hydraulic conductivity ($K_s$), as established in Chapter 8 of this guide. AS/NZS 1547 translates these soil categories into conservative loading rates (Tables L1 and M1), expressed in millimetres per day ($\text{mm/day}$), which is mathematically identical to litres per square metre per day ($\text{L/m}^2/\text{day}$).

+-------------------------------------------------------------------------+
|        MATHEMATICAL EQUIVALENCE OF SOIL LOADING UNITS                   |
|                                                                         |
|   1 mm depth of liquid applied over 1 m² of soil:                       |
|   Volume = 1 m x 1 m x 0.001 m = 0.001 m³                               |
|   Since 1 m³ = 1,000 Litres:                                            |
|   Volume = 0.001 x 1,000 L = 1.0 Litre                                  |
|                                                                         |
|   Therefore:  1 mm/day  =  1.0 Litre / m² / day                         |
+-------------------------------------------------------------------------+

AS/NZS 1547 Loading Rate Benchmark Matrix

The table below synthesizes the recommended Design Loading Rates (DLR) for absorption trenches/beds and Design Irrigation Rates (DIR) for subsurface drip irrigation across all six soil categories, contrasting primary and secondary treated effluent:

Critical Engineering Observations from the Matrix

1. Category 1 Soils (Coarse Sands/Gravels): While Category 1 soils exhibit immense hydraulic permeability, applying primary effluent is hazardous because liquid drains downward too rapidly ($>3.0\text{ m/day}$) before aerobic bacteria can oxidize pathogens. Pathogenic bacteria and viruses can reach unconfined drinking water aquifers within hours. Specialized design (or secondary treatment with a biological attenuating sand layer) is required.
2. Category 6 Soils (Heavy Clays): Primary effluent is strictly prohibited. The tight, microscopic pore structure and swelling clay minerals (such as montmorillonite/smectite) cannot absorb the high organic loading of septic tank effluent without forming an impermeable, airless biomat. Secondary treated effluent applied via shallow Subsurface Drip Irrigation (at a low rate of $2.0\text{ mm/day}$) or a Wisconsin Mound is the only compliant pathway.
3. Primary vs Secondary DLR Multipliers: Across Categories 3, 4, and 5, secondary treated effluent is permitted a DLR approximately $1.5$ to $2.0$ times higher than primary effluent. Because secondary effluent contains minimal $\text{BOD}_5$ and suspended solids, the soil interface maintains aerobic conditions and resists clogging, allowing higher daily loading.

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2. Mathematical Hydraulic Sizing Equations

Designing a compliant land application field requires a sequential four-step mathematical calculation.

+-------------------------------------------------------------------------+
|                   HYDRAULIC SIZING CALCULATION FLOW                     |
|                                                                         |
|   [STEP 1] Calculate Daily Design Flow (Q)                              |
|            Q = Design Occupancy (Persons) x Per Capita Allowance (L/p/d)|
|                                   |                                     |
|                                   v                                     |
|   [STEP 2] Calculate Required Basal Area (A)                            |
|            Area (m²) = Q (L/day) / DLR or DIR (mm/day)                  |
|                                   |                                     |
|                                   v                                     |
|   [STEP 3] Calculate Linear Pipe Length or Dripline Run                 |
|            Trenches: Length (m) = Basal Area (m²) / Trench Width (m)    |
|            SDI:      Length (m) = Irrigation Area (m²) / Spacing (m)    |
|                                   |                                     |
|                                   v                                     |
|   [STEP 4] Allocate 100% Reserve Area on Site Plan                      |
|            Reserve Area = Primary Area (m²)                             |
+-------------------------------------------------------------------------+

Step 1: Determining Design Wastewater Flow ($Q$)

The daily design wastewater volume is governed by the occupancy capacity of the dwelling and the water supply source. Under AS/NZS 1547:

• Occupancy Benchmark: Design occupancy is calculated on the basis of two persons for the master bedroom plus one person for each additional bedroom: $\text{Occupancy (Persons)} = \text{Number of Bedrooms} + 1$ (A 3-bedroom dwelling = 4 occupants; a 4-bedroom dwelling = 5 occupants; a 5-bedroom dwelling = 6 occupants). Sizing for fewer people because an elderly couple currently occupies a 4-bedroom house is an actionable design error; future purchasers will overload the system.
• Per Capita Water Allowances (AS/NZS 1547 Table H1/H2):
  • Mains Town Water Supply (Standard Fixtures): $180 - 200\text{ L/person/day}$.
  • Mains Town Water Supply (Full Water-Reduction Fixtures - 6/3L dual flush, aerated taps): $140 - 160\text{ L/person/day}$.
  • Roof Rainwater Tank Supply (Standard Fixtures): $140\text{ L/person/day}$.
  • Roof Rainwater Tank Supply (Full Water-Reduction Fixtures): $115 - 125\text{ L/person/day}$.

Step 2: Sizing Trench Basal Area

Under AS/NZS 1547, trench sizing is calculated on the horizontal basal floor area: $\text{Basal Area } A\text{ (m}^2\text{)} = \frac{Q\text{ (L/day)}}{\text{DLR (mm/day)}}$

Step 3: Determining Linear Trench Length

Once the basal area is established, the total linear trench length ($L$) is derived from the excavated trench bottom width ($W$): $L\text{ (metres)} = \frac{A\text{ (m}^2\text{)}}{W\text{ (metres)}}$

Basal Area vs Sidewall Area Infiltration

A longstanding debate in drainage design concerns whether sidewall area can be included in trench sizing. In conventional absorption trenches, effluent accumulates during peak discharge, flooding the sidewalls. AS/NZS 1547 maintains that primary sizing must be based 100% on the horizontal basal floor area. While aggregate sidewalls provide auxiliary infiltration and critical aeration during peak water events, relying on sidewalls for initial sizing reduces the safety factor and accelerates biomat failure when the basal floor experiences normal silting.

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3. Subsurface Drip Irrigation (SDI) Sizing & Emitter Hydraulics

Sizing an SDI field requires calculating the total square metres of irrigation area, the total linear metres of dripline, and the hydraulic output of the dripline grid to correctly match the dosing pump.

SDI Area Formula

$\text{Irrigation Area } A\text{ (m}^2\text{)} = \frac{Q\text{ (L/day)}}{\text{DIR (mm/day)}}$

Linear Tubing Length and Line Spacing

Dripline laterals are laid parallel across the contour of the slope at a regular line spacing ($S$). AS/NZS 1547 specifies spacing between $0.6\text{ metres}$ and $1.0\text{ metre}$:

• In Category 1-3 sandy and loamy soils, lateral spacing of $1.0\text{ metre}$ is common.
• In Category 4-5 loams and clay loams, lateral spacing is reduced to $0.8\text{ metres}$ to prevent dry bands between lines.
• On slopes exceeding $15\%$ or in Category 6 heavy clays, lateral spacing is tightened to $0.6\text{ metres}$ to ensure complete, uniform micro-distribution and prevent downslope channelling.

$\text{Total Dripline Length } L_{\text{line}}\text{ (m)} = \frac{A\text{ (m}^2\text{)}}{S\text{ (spacing in metres)}}$

Emitter Spacing and Hourly Field Discharge Calculation

To verify that the treatment plant's dosing pump and supply manifold can supply the field at the correct operating pressure ($100-350\text{ kPa}$), the certifying drainlayer must calculate the total field discharge rate ($Q_{\text{field}}$):

$\text{Total Number of Emitters } N = \frac{L_{\text{line}}}{\text{Emitter Spacing } (E_{\text{space}})}$ $Q_{\text{field}}\text{ (Litres/hour)} = N \times q_{\text{emitter}}\text{ (L/hr)}$ $Q_{\text{field}}\text{ (Litres/minute)} = \frac{Q_{\text{field}}\text{ (L/hr)}}{60}$

If the total field discharge rate exceeds the flow capacity of the pump at the required dynamic head, the drainlayer must divide the SDI field into two or more equal zones switched automatically via a sequencing valve (indexing valve) or motorized solenoid valves.

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4. The Mandatory 100% Reserve Area Requirement

Every regional council in New Zealand and AS/NZS 1547 Appendix L/M mandates that every on-site wastewater disposal design must incorporate an identified, protected, and fully compliant 100% reserve area.

+-------------------------------------------------------------------------+
|               SITE PLAN COVENANT: PRIMARY & RESERVE AREA                |
|                                                                         |
|   +-----------------------------------------------------------------+   |
|   |  PRIMARY DISPOSAL AREA (100% Active Field)                      |   |
|   |  - Active Subsurface Drip Irrigation or Absorption Trenches     |   |
|   |  - Current service lifespan: 15 to 25 years                     |   |
|   |  - Area: 220 m²                                                 |   |
|   +-----------------------------------------------------------------+   |
|                                                                         |
|   =================== NON-ENCROACHMENT BOUNDARY ====================   |
|                                                                         |
|   +-----------------------------------------------------------------+   |
|   |  RESERVE DISPOSAL AREA (100% Replacement Field)                 |   |
|   |  - Strictly protected, uncompacted natural soil                 |   |
|   |  - Identical soil category, setbacks, and hydraulic capacity    |   |
|   |  - NO BUILDINGS | NO DRIVEWAYS | NO VEGETABLE GARDENS           |   |
|   |  - Area: 220 m² (Guarantees future system replacement)          |   |
|   +-----------------------------------------------------------------+   |
+-------------------------------------------------------------------------+

Statutory Rationale & Regulatory Rules

1. Finite Soil Lifespan: Even perfectly designed and maintained land application areas have a finite operational lifespan, typically $15$ to $25$ years. Over decades, fine inorganic colloids, non-biodegradable silt, and dead bacterial biomass slowly reduce the macropore conductivity of the soil matrix. When the primary area reaches terminal clogging, a replacement field must be commissioned.
2. Preventing Lot Enclosure: Without an enforceable reserve area, property owners frequently construct auxiliary garages, asphalt driveways, in-ground swimming pools, or tennis courts across the remainder of their section. When the wastewater system inevitably fails, the property is trapped with zero compliant land, rendering the dwelling legally uninhabitable under the Building Act 2004 and the Health Act 1956.
3. Preservation of Soil Structure: The reserve area must remain pristine and uncompacted. Site plans submitted for building consent must explicitly mark the reserve boundary with prohibition covenants: no vehicle traffic, no contractor material stockpiles, no soil cutting or filling, and no deep-rooting invasive timber trees.

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5. Soil Clogging Dynamics and Biomat Equilibrium

Understanding the biochemical mechanism of biomat formation is essential for diagnosing failed systems and engineering resilient layouts.

+-------------------------------------------------------------------------+
|                 THE EQUILIBRIUM OF BIOMAT RESISTANCE                    |
|                                                                         |
|  Wastewater Infiltration                                                |
|       |                                                                 |
|       v                                                                 |
|  [ BIOMAT LAYER ] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
|  Anaerobic bacteria + extracellular polysaccharide slime + TSS          |
|  - Hydraulic conductivity drops from 1.0 m/day to 0.01 m/day            |
|  - Regulates downward flow velocity                                     |
|  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
|       |                                                                 |
|       v (Unsaturated, Aerobic Percolation)                              |
|  [ NATURAL SOIL PORES ]                                                 |
|  - Aerobic microbes consume remaining organics                          |
|  - Oxygen diffuses from surface to maintain oxidizing environment       |
|  - Viral and bacterial pathogens filtered and deactivated               |
|       |                                                                 |
|       v                                                                 |
|  Purified Groundwater Assimilation                                      |
+-------------------------------------------------------------------------+

The Dual Nature of the Biomat

1. The Healthy Biomat (Controlled Flow Regulator): When wastewater enters an aggregate trench, anaerobic bacteria feed on the organic matter in the effluent, secreting a viscous mesh of extracellular polysaccharides (slime) that coats the soil grains. This biological layer is called the biomat. A mature biomat exhibits a hydraulic conductivity several orders of magnitude lower than the clean subsoil. Counterintuitively, a controlled biomat is highly beneficial: by restricting infiltration velocity, it prevents wastewater from coursing rapidly through open soil cracks, forcing the liquid to spread across the entire trench base and percolate as a slow, unsaturated film through the aerobic subsoil below.
2. The Runaway Biomat (System Failure): If a septic tank is poorly maintained (unpumped sludge carrying over into the disposal field), or if the trench is hydraulically overloaded, the biomat thickens uncontrollably. Constant immersion deprives the trench floor of atmospheric oxygen. Anaerobic conditions predominate, black ferrous sulphide compounds precipitate into the soil pores, and the hydraulic conductivity plunges below the DLR. Wastewater ponds to the top of the aggregate, drowning the geotextile barrier and breaking out onto the surface.
3. Rejuvenation via Dual Alternating Fields: Where conventional trenches are installed, certifying drainlayers frequently install a dual alternating system utilizing a heavy-duty diversion valve (bull-run valve). The field is divided into two $100\%$ zones. Every 6 to 12 months, the drainlayer or owner turns the valve, diverting flow to the resting field. Deprived of moisture and nutrients, the starved biomat in the resting field undergoes aerobic desiccation and bacterial cannibalism. Earthworms and soil fungi break down the polysaccharide slime, fully restoring the natural hydraulic permeability of the soil floor.

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6. Comprehensive Worked Numerical Calculation: Sizing Primary LPED vs Secondary SDI

To master hydraulic calculations for the NZ PGDB Certifying Drainlayer Examination, examine this complete design scenario comparing two compliant pathways for a rural property in the Waikato region.

+-------------------------------------------------------------------------+
|                    DESIGN BRIEF & SITE PARAMETERS                       |
|                                                                         |
|   Dwelling: 4-bedroom contemporary home                                 |
|   Water Source: Roof rainwater tanks with full water-reduction fixtures |
|   Soil Classification: Category 5 Clay Loam (Well structured)           |
|   Design Saturated Permeability (Ks): 0.08 m/day                        |
+-------------------------------------------------------------------------+

Step 1: Calculate Design Wastewater Flow ($Q$)

$\text{Design Occupancy} = 4\text{ bedrooms} + 1 = 5\text{ persons}$ Under AS/NZS 1547 Table H1, per capita allowance for roof water with full water-reduction fixtures is $125\text{ L/person/day}$: $Q = 5\text{ persons} \times 125\text{ L/person/day} = 625\text{ Litres/day}$

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Option A: Primary Effluent with LPED in Gravel Trenches

1. Select DLR: For Soil Category 5 (Clay Loam) receiving primary septic tank effluent, AS/NZS 1547 Table L1 prescribes a DLR range of $5-8\text{ mm/day}$. Given the structured profile, select $\text{DLR} = 6.0\text{ mm/day}$ ($6.0\text{ L/m}^2/\text{day}$).
2. Calculate Required Basal Area ($A_{\text{basal}}$): $A_{\text{basal}} = \frac{Q}{\text{DLR}} = \frac{625\text{ L/day}}{6.0\text{ mm/day}} = 104.17\text{ m}^2$
3. Calculate Linear Trench Length ($L$): Specifying standard $0.6\text{ m}$ wide gravel trenches: $L_{\text{total}} = \frac{104.17\text{ m}^2}{0.6\text{ m}} = 173.61\text{ metres} \implies 174\text{ metres}$
4. Trench Network Configuration: Under LPED guidelines, maximum individual lateral run should be kept to $\le 20-25\text{ metres}$ to ensure uniform pressure distribution: $\text{Trenches required} = \frac{174\text{ m}}{22\text{ m/trench}} = 7.9 \implies 8\text{ trenches of } 21.75\text{ metres each}$
5. Reserve Area Allocation: Designate an additional $104.2\text{ m}^2$ of pristine, uncompacted ground alongside the primary trenches, clearly marked on the as-built plan.

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Option B: Secondary Treated Effluent with Subsurface Drip Irrigation (SDI)

1. Select DIR: For Soil Category 5 receiving secondary treated AWTS effluent, AS/NZS 1547 Table M1 prescribes a $\text{DIR} = 3.0\text{ mm/day}$ ($3.0\text{ L/m}^2/\text{day}$).
2. Calculate Required Irrigation Area ($A_{\text{irrigation}}$): $A_{\text{irrigation}} = \frac{Q}{\text{DIR}} = \frac{625\text{ L/day}}{3.0\text{ mm/day}} = 208.33\text{ m}^2 \implies 210\text{ m}^2$
3. Determine Lateral Line Spacing & Tubing Length: In Category 5 clay loam, lateral lines are spaced at $S = 0.8\text{ metres}$: $L_{\text{tubing}} = \frac{A_{\text{irrigation}}}{S} = \frac{210\text{ m}^2}{0.8\text{ m}} = 262.5\text{ metres}$
4. Calculate Emitter Count and Pump Output: Specifying pressure-compensating emitters spaced at $E_{\text{space}} = 0.5\text{ metres}$ along the tubing, with a discharge rate of $1.6\text{ L/hr}$ per emitter: $\text{Total Emitters } N = \frac{262.5\text{ m}}{0.5\text{ m}} = 525\text{ emitters}$ $Q_{\text{field}} = 525 \times 1.6\text{ L/hr} = 840\text{ Litres/hour}$ $Q_{\text{pump}} = \frac{840\text{ L/hr}}{60} = 14.0\text{ Litres/minute}$ Hydraulic Verification: The AWTS dosing pump must deliver at least $14.0\text{ L/min}$ at the design operating head (typically $15-20\text{ m}$ head to account for elevation, friction loss through 130-micron disc filter, and emitter backpressure).
5. Reserve Area Allocation: Designate an unencumbered $210\text{ m}^2$ reserve irrigation zone on the site plan.

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7. Trade Traps in Hydraulic Sizing and Reserve Planning

+-------------------------------------------------------------------------+
|                        DRAINLAYER TRADE TRAPS                           |
|                                                                         |
| [!] SIZING FOR CURRENT OCCUPANTS INSTEAD OF BEDROOMS                    |
| Sizing a disposal field for 2 retired residents living in a 5-bedroom   |
| luxury home (using 2 x 125 = 250 L/day instead of 6 x 125 = 750 L/day). |
| When the house is sold to a growing family of six, the disposal field   |
| experiences a 300% hydraulic overload, drowning the trenches in raw     |
| sewage within 90 days. Always size for full potential bedroom capacity. |
|                                                                         |
| [!] THE CONTRACTOR'S STOCKPILE PITFALL                                  |
| Failing to physically fence off the designated 100% reserve area during |
| house construction. The main builder stores 40 tonnes of gravel and     |
| parks heavy excavators across the reserve ground. The resulting subsoil |
| compaction destroys the soil macro-structure, slashing its permeability|
| by up to 80% before the house is even occupied.                         |
|                                                                         |
| [!] NEGLECTING SLOPE REDUCTION FACTORS                                  |
| Applying standard flat-ground DLR loading rates on slopes exceeding    |
| 10-15 degrees. On steep terrain, gravity induces rapid lateral sub-     |
| surface seepage, causing effluent to break out downslope above the next |
| trench line. AS/NZS 1547 mandates widening trench spacing and reducing  |
| loading rates on sloping ground to maintain hydraulic absorption.      |
+-------------------------------------------------------------------------+