Jun 15 2014
In Part II of “Water-The Third Rail”, the blog was a bit crude when saying
the Pt. Loma Wastewater Treatment Plant (PLWTP) takes the chunks out sewage
at the rate of two hundred million gallons a day, and dumps the rest in the
ocean. It is true. But we need to be more specific to understand what is
happening. To do that (recall I warned there would be eye-glazing detail
necessary) we need to use terminology that is not within the normal
vocabulary of non-wastewater engineers.
The PLWTP is a chemically-assisted primary treatment plant and is the
terminal treatment plant discharging to the Point Lorna Ocean Outfall
(PLOO). Translating, it is not a typical secondary treatment plant which
will be discussed below. As a “terminal plant”, no further treatment occurs
when it delivers its 200 mgd of effluent to a point 4.5 miles out beyond the
Pt. Loma coast, at a depth of 300 feet.
Although not dinner table talk, sewage is largely the product of human
digestion, having gone through the alimentary canals of millions of San
Diego residents. It contains organic material searching for oxygen. In a
secondary treatment plant (not at PLWTP), gaseous oxygen is pumped into the
flowing sewage after primary treatment to assist the aerobic bacteria whose
job it is to break down the organic material into non-oxygen seeking sludge.
At the PLWTP, the USEPA has provided a waiver for the past forty years [page 118] that
allows forty-two percent biological oxygen demand (BOD) sewage to escape any
further treatment before it leaves the Point Loma Ocean Outfall. The waiver
language uses a figure of fifty-eight per cent BOD removal. Granted, there
is a significant amount of monitoring to make sure the BOD is no higher than
42%. But that 200 mgd of sewage is still looking for oxygen.
And where does it find that oxygen? In the surrounding ocean, of course, where it competes with marine life.
In the 181 page USEPA waiver the engineers have provided a long list of
other criteria the plant must meet as the sewage becomes diluted with the
surrounding ocean water. Among those is oil and grease.
Usually referred to in the plumbing trade as FOG (fats, oils and grease),
the PLWTP cannot exceed (per the waiver) dumping more than twenty-one tons
of FOG into the ocean every day. Think about that. We all get nervous when
hear of an oil spill. In the course of a year, 7,800 tons of grease is
allowed by the waiver to be deposited 4.5 miles out from the Pt. Loma
coastline. The current USEPA waiver from the 1972 Clean Water Act allows this.
San Diego is doing what is considered really bad management practices in
homes and businesses. It is flushing fats, oils and greases down the toilet,
and has been doing this for four decades. Granted it’s just going into the
ocean and not clogging pipes, but is there much difference? Nice, huh? If
this blog has any technical errors, please comment.
By Milt Burgess • Blog • 0 • Tags: augmentation, California Aqueduct, Colorado River, conservation, El Nino, La Nina, MWD, purification, purple pipe, residential water user, SDCWA, State Water Project, water rate hike, water Storage