A conventional system needs permeable soil and enough depth above groundwater or bedrock. Where a site lacks either, the alternatives supply what is missing.
Mound systems
A drainfield built above the natural grade, in imported sand, with effluent pumped up into it.
Solves: shallow soil over bedrock, or a high water table — the mound creates the vertical separation the site does not have.
Requires: a pump, a large footprint, imported sand, and a visible landform in the garden.
Costs: several times a conventional field, and it needs the pump maintained.
Sand filters
Effluent is dosed onto a bed of sand, filtered and treated as it passes through, then dispersed to a smaller field.
Recirculating versions pass the effluent through repeatedly, giving high treatment quality.
Solves: poor soil, small lots, sensitive receiving environments. Because the effluent leaving is much cleaner, the dispersal area required is smaller.
Requires: a pump, a dosing control, and periodic media attention.
Aerobic treatment units
A tank with an air pump, growing aerobic bacteria that treat the effluent far more thoroughly than a septic tank's anaerobic process.
Solves: poor soil and tight sites, by producing effluent clean enough to disperse over a much smaller area.
Requires: continuous electricity, an air pump that wears out, and — importantly — a service contract in most jurisdictions, with scheduled inspections. Some require sampling.
Costs: high to install and, unusually for a septic system, meaningfully high to run.
Drip dispersal
Small-diameter tubing with pressure-compensating emitters, distributing treated effluent shallowly over a wide area.
Solves: sloping ground, irregular lots, shallow soil, and sites where the usable area is scattered rather than a single rectangle.
Requires: pre-treatment, filtration, a pump, and periodic field flushing.
Pressure distribution
Not a system type so much as a technique: a pump and laterals with small orifices, delivering effluent evenly across the whole absorption area.
Used with most of the above, and worth having on any sloping conventional field, where gravity would otherwise concentrate the flow at the low end.
Holding tanks
A sealed tank with no dispersal, pumped out periodically by a hauler.
Permitted only in limited circumstances in most jurisdictions, and expensive to operate — a household generates a great deal of wastewater. Generally a last resort or a temporary measure.
What they have in common
Mechanical components. Pumps, blowers, controls and alarms, all of which fail eventually and need power.
Maintenance obligations, frequently mandatory and sometimes contractually required as a condition of the permit.
Higher cost, both to build and over the life.
A greater consequence when neglected, since a system relying on treatment rather than soil has less margin.
Choosing
The site determines the shortlist, not preference. A soil evaluation and the local health department's rules narrow it quickly, and a designer familiar with that jurisdiction is worth the fee.
The question worth asking early, before land is bought: what does this lot actually require, and what does that cost? On a difficult site the answer can be a substantial fraction of the price of the land.
The operating obligation
The largest practical difference between a conventional field and an alternative system is not the installation cost. It is that somebody has to look after it.
Aerobic units in particular are commonly permitted on condition of a maintenance contract, with scheduled visits and sometimes effluent sampling reported to the health department. Letting that lapse can be a permit violation as well as a route to failure.
For a household choosing between a difficult lot with an alternative system and an easier lot with a conventional one, that ongoing obligation belongs in the comparison alongside the price.
Selling a house with one
A buyer's inspection of an alternative system looks at the mechanical components and at the maintenance record.
A system with a documented service history and a current contract transfers without difficulty. One with neither raises exactly the questions it should, and it is the kind of item that reduces an offer.