Secondary Treatment Systems
What is a secondary treatment sewage disposal system and why would I need to have such a system? This is a common question often posed by homeowners who are told that their property will require secondary treatment for onsite sewage disposal.
The Virginia Department of Health classifies sewage effluent (sewage
wastewater after it exits the septic tank or a sewage treatment device
in preparation for final disposal into the soil) as either "septic tank
effluent" or "secondary effluent". Septic tank effluent is classified
as residential strength wastewater having a five-day biochemical oxygen
demand (BOD) between 120 and 200 mg/l; total suspended solids (TSS)
between 70 and 150 mg/l; and fats, oils, and grease (FOG) of 30 mg/l or
less. Septic tank effluent is typically treated by
disposal via a drainfield or low pressure distribution into natural
soil at least 18" above the seasonal water table (Virginia
Regulations). Naturally occurring aerobic bacteria and other beneficial
microorganisms in the soil feed on the harmful sewage bacteria and
dissolved organics before they percolate down into the water table.
When there is less than an 18" stand-off to the water table, adequate treatment may not occur. In these instances, treatment must be accomplished through specially designed treatment devices and this form of treatment is referred to as "secondary treatment".
Secondary
treatment effluent is treated to reduce the BOD to 30 mg/l or less; TSS
to 30 mg/l or less; and FOG to less than 5 mg/l. Secondary treatment systems
typically are either aerobic treatment units (ATUs) or filter systems.
ATUs oxygenate the effluent to allow the growth of aerobic microorganisms
that in turn decompose dissolved organic and nitrogen compounds into CO2
or inorganic compounds. Filter systems such as sand and peat filters use
filtration media to both filter and treat the effluent through special
bio-chemical interactions of the effluent with naturally-occurring chemicals
and microorganisms within the filter media.
At Affordable Septic Solutions, we work with our clients to provide them with the latest in available treatment options and help them determine which is the best system for their particular property's unique site and soil conditions so that they can make an informed decision in choosing the treatment system most suitable to their needs. Through initial consultations to design to final permit approval, we work with our clients to make the permitting process as painless and as swift as possible. Our staff of field specialists also work with sewage system contractors to see that our permit designs are installed in accordance with our plans and specifications and/or those of professional engineers that we collaborate with on commercial and multi-family projects.
