The 7 com­mon rea­sons why a sep­tic tank fails

The ma­jor­ity of sep­tic tank sys­tems are old, and a high num­ber are at risk of fail­ing.

NZ Lifestyle Block - - Notebook -

It’s es­ti­mated that more than 40,000 of the 270,000 or so on-site do­mes­tic waste­water sys­tems in NZ are at risk of fail­ing.

Fail­ure rates of on-site sys­tems are es­ti­mated to range from 15-50 per cent, which equates to be­tween 40,000-130,000 fail­ing sys­tems na­tion­ally. There is a large amount of vari­abil­ity in the es­ti­mates due to the vari­a­tion in lo­cal fac­tors, in­clud­ing ge­ol­ogy, cli­mate, de­sign and in­stal­la­tion, lot size, and the age of the com­mu­nity.

In a re­port on do­mes­tic waste­water sys­tems, the Min­istry for the En­vi­ron­ment wrote most peo­ple weren’t aware that to keep a sys­tem work­ing well re­quired reg­u­lar in­spec­tions and on­go­ing main­te­nance, while a few weren’t even aware they had a sep­tic tank sys­tem.

A se­lec­tion of in-depth sur­veys by re­gional, dis­trict and city coun­cils in the mid-2000s high­lighted the over­all bad per­for­mance of on-site sys­tems.

• A sur­vey of 3251 sys­tems in the Bay of Plenty found that 64 per­cent of the sys­tems sur­veyed failed an in­spec­tion.

• A sur­vey around Lake Ro­torua found that 77 per­cent of sep­tic tanks within the Ro­tokawa/brunswick area did not com­ply with the En­vi­ron­ment Bay of Plenty. Ninety per­cent of own­ers did not clean their on-site sys­tems once per decade, con­tribut­ing to the high nu­tri­ent load in Lake Ro­torua. Wa­ter qual­ity within streams and springs in the area showed high lev­els of fae­cal con­tam­i­na­tion.

• In­spec­tions of 2000 prop­er­ties on Wai­heke Is­land (Auck­land City Coun­cil) in­di­cated that around 11 per­cent had mi­nor prob­lems and a fur­ther 3 per­cent had ma­jor prob­lems.

• An as­sess­ment of on-site sys­tems in Cleve­don Vil­lage, Manukau, found that ap­prox­i­mately 20 per­cent of on-site sys­tems were sub­ject to fail­ure at the time of the in­spec­tion and a fur­ther 10 per­cent were con­sid­ered po­ten­tially likely to fail.

Fail­ure is de­fined as the sit­u­a­tion where in­ad­e­quately treated waste­water en­ters ground­wa­ter or sur­face wa­ter, cre­at­ing an en­vi­ron­men­tal risk, or rises to the ground sur­face, cre­at­ing a risk to hu­man health. This can oc­cur through:

• in­ad­e­quate man­age­ment of the sys­tem (eg, dis­pos­ing of un­suit­able items or chem­i­cals);

• in­ad­e­quate main­te­nance of the sys­tem (eg, not pump­ing out the tank when re­quired);

• the sep­tic tank leak­ing di­rectly into the ground through cracks in the tank walls and joints;

• the on-site sys­tem be­ing con­nected, ei­ther in­ten­tion­ally or by ac­ci­dent, to stormwa­ter pipes or open stormwa­ter drains, lead­ing to over­load­ing;

• the pipes in the dis­posal field be­com­ing blocked, caus­ing con­cen­trated waste­water to dis­charge into the ground;

• the dis­posal field soil not be­ing per­me­able enough, caus­ing waste­water to rise to the ground sur­face (run-off to sur­face wa­ters or dis­charge di­rectly into ground­wa­ter through large cracks in the soil is pos­si­ble);

• the dis­posal field soil be­ing too per­me­able (eg, coarse sands or grav­els), al­low­ing the waste­water to en­ter ground­wa­ter with­out ad­e­quate treat­ment in the un­sat­u­rated soil (re­moval of con­tam­i­nants such as pathogens is much more ef­fec­tive in un­sat­u­rated soils);

• the dis­posal field be­ing too close to the ground­wa­ter ta­ble (in high ground­wa­ter sit­u­a­tions), al­low­ing the waste­water to en­ter the ground­wa­ter with­out ad­e­quate treat­ment (con­tam­i­nated ground­wa­ter can then flow into sur­face wa­ters, con­tam­i­nat­ing those sur­face wa­ters);

• the sys­tem not hav­ing enough ca­pac­ity for the size of the dwelling.

Most peo­ple weren’t aware they needed to do reg­u­lar in­spec­tions and on­go­ing main­te­nance

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