What wastewater system is best for a large family?

You might be building a five-bedroom home for two adults and three children. Or maybe there are only three of you, and the extra rooms are for guests. Either way, the wastewater system needs to account for the home’s potential use. In other words, a five bedroom home needs a wastewater system that can cope with 10 people (2 per room). 

Why? Because the people living there may change. Children grow up, relatives move in, and a future owner might fill every bedroom. A system designed around today’s household could leave the next household short.

So, bedrooms are used by designers and council to estimate the number of people the home could accommodate. That figure is then used to calculate how much wastewater the system needs to manage each day. Some councils also consider other rooms that could reasonably become bedrooms (like a home office). 

Daily wastewater flow is the amount of used water going from your house into the wastewater system over a day, measured in litres.

It includes water from:

• Toilets.

• Showers, baths and bathroom basins.

• The kitchen sink and dishwasher.

• The laundry.

Your designer calculates a design daily flow: the amount the system must be designed to manage. That calculation uses the home’s design occupancy and an appropriate daily wastewater allowance per person.

For example, if a design allowed for eight people at 180 litres per person per day, the daily design flow would be 1,440 litres.

That is an illustration, not a fixed allowance for every home. Your actual design figure depends on the applicable requirements, water supply, fixtures and intended use.

You don’t need to do the calculations yourself. But it is worth asking your supplier:

“How many litres a day is this system designed to treat, and what have you based that on?”

That tells you much more than “it’s a big tank”.

Tank volume tells you how much a tank holds. Daily treatment capacity tells you how much wastewater the system is designed to process each day.

A tank holding several thousand litres doesn't necessarily treat that same number of litres every day. Wastewater needs time to move through the treatment stages, and space may also be needed for solids, storage and managing incoming flows.

Think of it this way: the size of your kitchen doesn’t tell you how many dinners you can cook in an hour.

When comparing systems, look at the designed daily flow and treatment performance, alongside tank size.

Family water use rarely arrives in a steady, polite trickle.

There’s the morning rush. Then the washing machine goes on. Someone has a bath, the dishwasher starts, and suddenly quite a lot of water is heading in the same direction.

These short bursts are called peak flows.

A suitable design needs to account for both:

• The total daily flow: how much wastewater arrives over the whole day.

• The peaks: how much arrives during a shorter, busy period.

Depending on the system, storage and controlled pumping can help manage those bursts so wastewater moves through treatment and into the disposal field at an appropriate rate.

Tell your designer about regular overnight guests, several bathrooms being used together, unusually high water use or big family gatherings. A house that hosts Christmas lunch is one thing. A property hosting a crowd every weekend is another.

The aim is a system that fits normal family life, without needing a shower roster on the fridge.

As an early planning guide, we often allow around 100 m² per bedroom for a wastewater irrigation field.

Using that guide:

These are planning estimates, not final design sizes or a nationwide council rule.

The actual area depends on how much wastewater needs to be discharged, the soil’s ability to absorb it, the treatment level, slope and the disposal method.

You may also need room for separation distances from boundaries, buildings, waterways and other features, plus a reserve area where required. Those requirements can affect how much usable land you need beyond the field itself. 

This is why it pays to plan wastewater early. The house, driveway, shed and pool shouldn’t get every good spot before the disposal field gets a look-in.

A large dripline field isn’t the only option. If space is tight, your designer can investigate other land application methods, including soakage beds, trenches or an appropriately designed evapotranspiration bed.

A conventional soakage bed distributes treated wastewater into suitable ground through a prepared bed. Where the soil and site conditions allow, it can use less land than an irrigation field.

But a smaller footprint has to suit the ground. Conventional soakage beds depend on sufficiently free-draining soil. They aren’t an automatic solution for heavy clay, high groundwater or a flood-prone area. 

The useful question is:

“Which disposal method will safely manage our wastewater in the space we have?”

Once that is understood, the treatment system can be matched to it.

NaturalFlow, BioPod and EconoTreat can all be sized to suit larger households. Having a big family does not automatically make one brand the best choice.

The right selection depends on four things.

1. Your land

Soil, slope, groundwater and nearby waterways influence how wastewater can be treated and discharged.

A property with well-draining soil may have different options from a clay site with limited usable ground.

2. Your available space

The design needs room for both the treatment system and the disposal field, along with access for installation and future servicing.

A tank that fits neatly beside the house is only part of the answer. The treated water still needs somewhere suitable to go.

3. Your lifestyle

You might prioritise low electricity use, an off-grid setup, particular maintenance arrangements or accommodating regular visitors.

NaturalFlow’s treatment process operates without electricity, although pumps may still be needed depending on the site. BioPod and EconoTreat have their own operating and servicing requirements to consider.

Compare the complete installation and its ongoing needs, rather than choosing on a product name alone.

4. Your council requirements

The proposed system must meet the treatment and discharge requirements that apply to your council.

A well designed system should accommodate the normal ups and downs of household use. You won’t produce exactly the same amount of wastewater every day, and the design should recognise that.

There are still a few simple ways to help it along:

• Spread several loads of washing across the week where practical.
• Fix leaking toilets and taps promptly.
• Keep wipes, sanitary products and cooking fat out of the drains.
• Follow the supplier’s advice on cleaning products and servicing.
• Tell your designer about extended absences or major seasonal changes in occupancy.

You don’t need to become the household water police. The aim is to avoid unnecessary loads and give the system the conditions it was designed for.

Mention future plans before the wastewater design is finalised.

Extra bedrooms, a minor dwelling or additional accommodation can increase the design flow. They may require more treatment capacity, a larger disposal area or changes to the consent.

Even if the new building is years away, planning now can help you protect suitable land and understand what an upgrade would involve.

“Large enough for our house” and “large enough for two homes” are different briefs.

You should come away from a wastewater proposal understanding:

• How many bedrooms and what design occupancy it allows for.
• How many litres a day it is designed to treat.
• How it manages busy periods and fluctuating use.
• Which disposal method is proposed and why it suits the land.
• How much space the field, setbacks and any reserve area need.
• What power, servicing and maintenance it requires.
• Whether the quote includes design, consent work, installation and the complete disposal field.

The best system for a large family is the one that fits the whole property and the way it will be used. Start with the bedrooms and daily flow, work out what the land can manage, then choose the brand and configuration that fit.