Summary:

Untreated domestic sewerage still poses a serious threat to the environment in rural
areas. The lack of simultaneous construction of water supply and sewerage systems
in rural areas contributed to the significant imbalance between water supply and sewage
discharge. Raw sewage, entering the natural receiver, cause rapid degradation of
life and contribute to the biological imbalance in the environment. Untreated sewage
can cause the deterioration of water quality in private wells and may even lead to its
total contamination.
In areas, where it is impossible to implement collective sewerage systems, there is
an alternative in the form of building septic tanks or household sewage treatment plants.
The development of household sewage treatment plants in Poland has been observed for
decades. There were 4000 household sewage treatment plants registered in 1998, while
in 1999 this number increased to 18054. A rapid increase was observed in subsequent
years, i.e. in 2008 there were 51,943 registered household sewage treatment plants, in
2010 this number increased to about 81 thousand and in 2011 about 103 thousand
treatment plants were registered. In the near future a dramatic increase in the number
of newly built household sewage treatment plants is expected.
The aim of this study was to analyze the efficiency of domestic sewage treatment
in a household sewage treatment plant with a modified vertical flow sand and gravel
filter. The aim was achieved by performing preliminary laboratory studies using a fragmentary
"M1" model in a scale 1:1, and then by construction of a modified prototype of
"F1" filter in the field and determining its effectiveness under field conditions. On this
basis, the author proposed an innovative solution - a modified vertical flow gravel and
sand filter. Currently, the procedures related to the patent application of the prototype
of the modified gravel and sand filter are being underway.
The original solution in the vertical flow sand filter consisted in:
- placement of a protective layer of small pebbles before the filtration (sand)
layer,
- application of innovative sewage distribution system in the form of the proprietary
solution,
- reduction of the filter surface as compared to conventional solutions.
The classic vertical flow sand filters occupy a significant surface (25-30 m² for a
five persons household). This study attempts to reduce the dimensions of the filter on the
plan. An important element of the bed's modification is the design of a suitable sewage
distribution system in order to utilize the filter layer as much as possible. The spacing of
sewage distribution drains on the surface of the filter bed in conventional solutions is
very large and ranges up to 200 cm. With thickness of the proper filtration layer from 60
to 100 cm the filter bed may be unevenly loaded. By increasing the use of the filter layer
and introducing an additional protective layer (made of small pebbles) one can significantly
reduce the dimensions of vertical flow sand filter while at the same time maintaining
the acceptable level of the filtrate quality.
The study included the determination of physical and chemical indicators of pollution
in sewage flowing into and out of the filter: BOD₅, COD_Cr, dissolved oxygen, totalsuspended solids, pH, total phosphorus, ammonium nitrogen NH4, organic nitrogen,
total Kjeldahl nitrogen, nitrite nitrogen NO2, nitrate nitrogen NO3 and total nitrogen
(the values of physical and chemical indicators of sewage collected from the following
depths of the filter bed were determined: 10 cm, 30 cm, 50 cm, 90 cm). Moreover, the
abundance of microbial indicators of sewage pollution (coliform bacteria, Escherichia
coli, Enterococcus faecalis, Salmonella spp., Shigella spp. and Clostridium perfringens)
was determined in sewage flowing into the filter, at the outflow from the gravel layer
and at the outflow from the sand layer.
Based on the results of the analyzes it is recommended to use a protective layer
made of small pebbles with a normal grain size of d10=1.66mm and thickness of 50 cm.
Under this layer there should be a proper filtration layer made of rinsed sand with normal
diameter of d10=0.40mm and thickness of at least 60 cm. An important advantage of
the modified vertical flow gravel and sand filter is its small surface (1.15 m2.M-1) while
in the classical solutions it is even up to 7 m2.M-1. Taking into consideration that the
removal of pollutants is most intense at the initial depth of the protective layer, it is very
important to evenly distribute the pre-treated sewage over the surface of the protective
layer made of small pebbles. The use of innovative sewage distribution system allowed
the optimal use of the filter bed's surface during the study period. Due to the large
amounts of biogenic compounds in treated sewage, the use of the modified vertical flow
gravel and sand filter is not recommended when discharging treated sewage into stagnant
water bodies and lakes.