The resident count in a treatment unit description looks like a simple selection criterion: four people live in the home, so buy a four-person unit. In practice, counting heads alone does not show when or how much wastewater enters the system. Distinguish average daily flow, pollutant load and short-term usage peaks. This article supports informed selection rather than replacing a designer's calculation with one online formula.
Residents and population equivalent are different concepts
Resident count describes the household; population equivalent describes load. Two equal-sized homes may use water differently: one houses a permanent family, another is occupied only at weekends with guests. Neither floor area nor bedroom count alone reveals the future wastewater pattern.
The official August design information lists AT6 as 4 GE and 0,6 m³ per day, and AT8 as 6 GE and 0,9 m³ per day. This shows why a model number cannot directly establish resident capacity. These are specific models' documented figures, not a universal naming rule across manufacturers.
Product ratings do not replace design requirements
An important limit: a manufacturer's table alone does not confirm suitability for a new single-family home in Lithuania. The current Environmental Protection Department guidance states that design capacity for a single-family home must reflect actual or forecast conditions and be no less than 4 GE, 180 l/h and 0,8 m³/d. Consider these together rather than selecting only the most convenient figure. The AT6 rating above illustrates document reading; it is not advice to select that model automatically for a single-family home. The designer justifies suitability for the specific property under applicable requirements.
Count more than the monthly average
Water meter data is a good starting point. Record readings on several normal and high-use days. Note irrigation, outdoor taps and other water not entering domestic wastewater separately. Treating the entire metered volume as treatment-unit load misrepresents the situation. Conversely, an additional water source may be missed in the overall calculation.
An illustrative example. Weekly water records show 4,2 m³. Dividing by seven gives 0,6 m³ per day but does not show whether almost half was used on Saturday. That average does not authorise selecting a unit at its limit. Show the specialist daily distribution and the habits responsible for the highest-use day.
Assess hydraulic and organic loads separately
More water does not necessarily mean a proportional pollutant quantity, while concentrated domestic wastewater can impose substantial organic load at lower volume. The proposal should show both parameters, not merely an attractively presented cubic-metre figure. The AT declaration of performance states nominal flow and organic load as separate characteristics.
Owners need not measure every daily discharge in a laboratory. Their task is to explain use: permanent residence, rented holiday home, frequent gatherings or additional activities. If the property is not an ordinary home, describing it simply as a 'six-person house' is overly simplistic.
Prepare a usage calendar for selection discussions
It is useful to sketch out a simple week. For each day, note how many people stay overnight, whether laundry is done, when the bath is used and whether guests visit. There is no need to turn every tap into a separate spreadsheet. The aim is to show the specialist the situations that a monthly bill alone does not reveal.
| Situation | What to tell the designer |
|---|---|
| Permanent residence | Normal and maximum occupant numbers |
| Weekend property | Visit frequency and periods without use |
| Rental | Possible occupancy and changeover days |
| Planned expansion | Additional building or bathroom |
If future demand is unclear, label it as an assumption. Several scenarios can then be compared instead of pretending usage five years from now is precisely known today.
Why 'bigger is better' does not always help
A larger vessel looks like an attractive reserve, but selecting a biological process involves more than storage volume. Manufacturer operating conditions, low-load operation and realistic maintenance matter. Buying a larger model does not resolve unsuitable discharge arrangements, groundwater or electricity supply.
Ask why the particular model is proposed and which usage assumption was applied. 'Everyone installs this one' is not selection logic. The proposal should identify model, documentation links and an explanation of normal and maximum load assessment.
Unexpected flows must not become a hidden reserve
Before selection, establish where roof water, drainage and other equipment discharges go. Do not simply add them to average domestic use and assume a bigger unit solves everything. Discuss water-treatment backwashing and other unusual flows separately. Assess compatibility for the particular equipment and treatment technology.
This is especially important at older properties with long-established connections and no schematic. Actual routes may need identification before ordering a new product. A simple clear schematic may offer more benefit than buying extra capacity for an unclear system.
Reserve capacity needs a clear purpose
Agree what 'reserve' means during selection. One person means another permanent resident; another means twenty guests at a summer gathering. These differ, and a percentage allowance alone does not explain them. Describe how long higher load may last and how often it recurs. If unknown, provide two separate scenarios and request conclusions for both.
Read proposal notes with the designer before signing. An unconfirmed wish for expansion must not silently become a promise that the system suits any increase. A clear limit allows timely reassessment rather than assigning blame after use has already changed.
What answer should the proposal provide?
A proper selection outcome should include the user scenario, documented model, daily and short-term load assessment and necessary operating conditions. Ask what must be reviewed if resident numbers change or another building is added. This helps avoid disputes about whether the original proposal covered later expansion.
We discuss indoor drainage planning in the plumbing section, and for a specific plot you can submit an enquiry. Include the real usage schedule, not only occupant count. A clearer household description makes different proposals more meaningfully comparable.
Information sources
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