Short definition
Sand filter and mound systems are engineered alternatives to standard in-ground drainfields, used where the soil can’t take effluent the conventional way: high water table, shallow soil over rock or hardpan, or slow-percolation clay. A sand filter is an intermediate-treatment chamber between the tank and the drainfield. A mound is a raised bed of sand and topsoil built above grade, with effluent pressurized-dosed through perforated laterals at the top.
What it is
Sand filter: A buried or above-ground chamber filled with graded sand sits between the septic tank and the drainfield. Effluent is dosed onto the sand, percolates through, and is collected at the bottom for further dispersal. The sand provides additional treatment so the downstream drainfield can be smaller or used in worse soil.
Mound system: Instead of trenching down into the soil, the system builds up. A bed of sand and topsoil is constructed above existing grade, and effluent is pressurized through perforated laterals at the top of the mound. The mound provides separation from a high water table or shallow rock.
Both are classified by WAC 246-272A as alternative on-site sewage systems — they require a licensed designer, additional permitting, and annual inspection (vs. every-3-years for gravity systems).
Why it matters to a homeowner
Cost is the headline:
- Sand filter or mound install: $20,000–$45,000+ — significantly more than gravity septic ($4,000–$8,000) or ATU ($10,000–$25,000).
- Maintenance contract typically required.
- Annual inspection required by WAC 246-272A.
If you’re buying a property with a sand filter or mound system, the long-term ownership math is meaningfully different from a standard gravity-septic property. If you’re building on marginal soil, this might be the only legal option — and the design needs to come from a licensed WA designer who knows your county’s specific requirements.
When you’ll encounter this term
- Building on Olympic Peninsula or San Juan Islands marginal soil — county health may require a mound or sand filter from day one.
- Existing drainfield failed and the soil is too marginal for a standard replacement — mound becomes the only legal option.
- Property purchase: the original system permit shows a mound or sand filter; ask about maintenance contract status.
Common variants / not the same as
- Sand filter vs. mound. Sand filter is intermediate treatment between tank and drainfield. Mound is intermediate treatment plus disposal in one above-grade bed.
- Mound vs. ATU. Mound is a soil-bed (gravity-fed or pumped). ATU is a mechanically-aerated tank — different stage of treatment, sometimes paired together on tough sites.
- Mound vs. drip distribution. Drip uses small-diameter tubing for very even effluent dispersal, often paired with an ATU for marginal sites.
Common failure modes
- Pump failure on the dosing pump → effluent backs up; high-water alarm should trigger.
- Distribution laterals freezing in eastern WA winter → temporary failure.
- Sand filter media plugging over time (10–15 years) → media replacement needed.
- Mound surface erosion or settling → re-grading required.
Washington note
Alternative systems are concentrated in Mason, Kitsap, Jefferson, Clallam, San Juan, Skagit, Whatcom counties — areas with marginal soils, high water tables, or sensitive shoreline/shellfish-protection requirements. Annual inspection and maintenance contracts are typically required. WA Department of Health certifies designers and installers; per-county design standards vary, so confirm before specifying.