Apparatus and method for monitoring the condition of septic tanks

An electronic sensor for detecting when a septic tank reaches it's design capacity. The sensor uses a sonar transponder to measure the thickness of the three layers of material present in a properly operating septic tank. The sonar transponder is encapsulated in a environmentally sealed sensor assembly that floats in a self-orienting fashion at the top of the material contained in the septic tank. The sensor relays signals through a cable harness to a signal interpretation module located in a convenient indoor location. The signal interpretation module contains a digital signal processing (DSP) sub-system and an alarm sub-system. The DSP sub-system interprets the electronic signals from the sonar sensor resulting in a measurement of the relative volume of the material in septic tank as well as measures of the elevation of the bottom of the top (scum) layer and the top of the bottom (sludge) layer. Any of these measures may indicate that the tank has reached it's design capacity and should therefor be pumped. The alarm sub-system provides visual and audio alarms in a escalating fashion providing early warning of the impending need to service the tank.

FIELD OF THE INVENTION 
The present invention relates to septic tank monitoring systems, and more 
specifically to such systems which provide for remote, electronic 
notification and control. 
DESCRIPTION RELATIVE TO THE PRIOR ART 
This invention improves on septic system design by monitoring the condition 
of the septic system, thereby providing timely indication of when it is 
necessary to empty the septic tank. 
Septic systems consist of a septic tank in which wastes are collected, 
settled, and partially digested connected to a drain field which disperses 
the resulting "gray" water. In a properly operating septic system this 
gray water will contain little or no suspended solids. Furthermore, the 
presence of suspended solids in the gray water will, over time, result in 
the "failure" of the drainage field. This failure is a result of the field 
becoming clogged by the solids to such an extent that it can no longer 
absorb and disperse the gray water. This failure will typically be 
evidenced by pollution of ground and surface water. This can be an 
expensive, or in rare cases (typically for regulatory reasons), 
impossible, problem to fix. 
A septic tank contains three biologically active zones. Waste matter enters 
the middle "liquid" zone. Heavy solids then settle to the bottom of the 
tank as sediment, or sludge, where they are further decomposed, although 
some of the sediment will not be biodegradable. Fats and other lighter 
suspended solids rise to the top of the tank forming a "cake", or "scum" 
which may also undergoes further decomposition. The design of the septic 
tank insure that, under normal operation, only material from fluid middle 
layer of enters the drainage field. The settlement rate in a tank is a 
function of the effective volume of the tank and rate of flow. In this 
case the effective volume of the tank is the volume of the middle liquid 
zone. This volume establishes a design capacity of the tank; that is, the 
ability of the tank to process the material flowing in at a particular 
rate. If the inflow rate exceeds a certain amount, the tank will be unable 
to process the material within fast enough to prevent failure of the 
system. 
Distinguished from design capacity is the system capacity. The system 
capacity is the ability of a given system to continue to process more 
material. The system capacity reaches zero when one of several things 
happens: 
a) particles of the sediment layer or cake layer are allowed to exit the 
system through the outflow; or 
b) the sediment layer and the cake layer approach each other to the point 
where little if any of the liquid layer remains. 
During the course of normal operation of a septic system, both the bottom 
layer and the top layer are continually augmented by new material from the 
input waste stream. The material in both the top and the bottom layers 
undergoes anaerobic decomposition; thus the accumulation rate is 
substantially less than the rate at which corresponding solids are added 
to the system. Even with this decomposition, there is a gradual increase 
in the volume of both the top and the bottom layers over time. 
Periodically a septic tank must be pumped of these accumulated solids. 
Pumping is indicated when the volume of the central fluid portion is down 
to about 33% of the total of the three layers. Modern septic designs 
provide for several years between pumping; however eventually it will be 
required for any system being actively used. The interval between pumping 
is a function of the waste mix, volume of waste, and the effectiveness of 
the biological decomposition in the septic tank. Pumping may be indicated 
by 1) The absolute location of the top layer, 2) The absolute location of 
the bottom layer, or 3) A combination of these factors that reduced the 
volume of the central layer sufficiently. As the volume of solids 
increases in a tank, the effectiveness of the biological decomposition 
typically increases, resulting in a decreased rate of accumulation for a 
constant input rate. Taking into account all of these factors, it is 
virtually impossible to predict a priori when a tank will actually require 
pumping; experience indicates that the pumping interval can range from 2 
to 15 years. 
As the tank accumulates solids, the settlement effectiveness of the tank 
decreases resulting in increased discharge of solids into the gray water 
stream. Often the system has failed, in other words, begun to discharge 
substantial solids into the gray water, before there is a visible sign of 
failure to the operator. Typically, the first sign that the system 
operator (typically a homeowner) sees is an unsafe and unsanitary backup 
of waste material into the residence. By the time this happens, it is 
likely that substantial suspended solids have been released into the drain 
field. This "invisible" failure of septic systems is addressed by this 
invention. 
The classical means of evaluating whether a septic system requires pumping 
is to remove the access cover to the septic tank and to measure the depth 
of each of the three layers by pushing a "flapper stick" into the tank. 
The flapper stick is configured so that it can be used to gauge the bottom 
of the cake. Direct observation of the stick after removal indicates the 
depth of the bottom sediment. This is an intrusive (landscaping is 
typically disturbed as the tank is opened) and unpleasant task. In 
practice it is seldom done by septic system operators. This invention 
improves on this process by providing effectively the same measurements in 
an automatic and on a continual basis. 
A search of the prior art reveals a number of devices that measure or 
monitor septic tank contents in less desirable ways and devices that 
control the discharge effluent so as to limit the egress of undesirable 
particulate matter. The current invention represents an improvement over 
these previous devices since monitoring is continuous, non-invasive, 
automated, and provides an escalating level of feedback prior to any 
failure symptoms. It also improves on these devices by monitoring 
accumulation of solids at both the top and the bottom of the tank. A 
further improvement over some of the prior art is that the system 
described is practical for application in residential and anaerobic 
commercial waste disposal systems. A further improvement is that the 
device is easily installed in both new and existing septic tanks and that 
it does not interfere in any way with pumping operations. 
The Patent to Wilkerson, U.S. Pat. No. 3,954,612 describes a mechanical 
device for measuring the height of the sludge in a septic tank. The device 
consists of a float assembly that is designed to ride at the interface 
between the fluid layer and the bottom sludge layer. The float has a 
mechanical arm that exits through the top of the septic tank allowing for 
direct observation of the sludge depth under the float. The current 
invention, an electronic device, is a marked improvement in reliability 
and accuracy over mechanical systems of the Wilkerson type. 
The Patent to Anderson, U.S. Pat. No. 4,319,998 issued Mar. 16, 1982 
describes a monitor for the gray water effluent. The monitor is inserted 
in-line between the septic tank and the drainage field. The device 
measures a total accumulation of suspended solids in the effluent stream 
and is designed so that a switch can be triggered resulting in remote 
notification. Anderson's patent is basically a mechanical monitoring 
system like Wilkerson's invention. 
The Patent to Bowman, U.S. Pat. No. 4,715,966 issued on Dec. 29, 1987 
describes a device intended to measure the thickness of sludge in a tank. 
The device consists of a float assembly that is designed to ride at the 
interface between the fluid layer and the bottom sludge layer. The float 
has a mechanical arm that exits through the top of the septic tank 
allowing for direct observation of the sludge depth under the float. This 
is another mechanical device similar to the Wilkerson Patent. 
The Patent by Norcross, U.S. Pat. No. 5,421,995 describes a device for 
controlling the decanting of clarified liquid from a batch reactor. 
Norcross is not directly applicable for septic systems, as these systems 
are not batch reactors. The Norcross device consists of a mechanical 
assembly linked to the outlet. The assembly is essentially a hinge 
mechanism with one end fixed at the outlet and the other end floating. A 
key component of the invention is an electronic sensor suspended from the 
floating end that detects the sludge blanket. Norcross mentions the use of 
sonar as an alternative sensor, but does not develop this idea to any 
significant degree. It is believed that the optical sensor disclosed in 
Norcross does not operate reliably in the presence of solids or optically 
dense material in the septic tank which interfere with the system optics. 
The current invention does not suffer from these drawbacks. 
SUMMARY OF THE INVENTION 
It is the general object of this invention to provide an apparatus that 
indicates when a septic tank needs to be pumped. A specific object of this 
invention is to provide such an apparatus such that the apparatus can 
provide quantitative feedback allowing a septic system operator to assess 
on an ad-hoc basis how near the tank is to needing to be pumped. It is a 
further specific object of this invention that such monitoring be done 
without requiring opening of the tank (other than for the initial 
installation of the sensor assembly) and without the need for any physical 
measuring device to protrude from the tank. 
According to one aspect of the invention, an apparatus is disclosed for 
monitoring the status of a septic tank of the type which contains a sludge 
level, liquid level, and a scum level. The invention includes means for 
sensing the location of the boundary between the sludge level and the 
liquid level, means for sensing the location of the boundary between the 
liquid level and the scum level, and means for calculating the status of 
the tank based on these locations. There are, furthermore, means for 
displaying said status, so that the user of the septic tank can determine 
whether the septic tank requires interventive action. 
According to a second aspect of the invention, the septic tank has a design 
capacity, and the means for displaying further includes means for 
displaying whether or not the septic tank has reached its design capacity. 
According to a third aspect of the invention the means for sensing further 
includes electronic means. 
According to still another aspect of the invention electronic means further 
includes sonic means, and the means for displaying includes remote means. 
According to yet another aspect of the invention, there are also provided 
means to input septic tank dimensions into said means for calculating, as 
well as means to calculate the relative volumes of the three levels, and 
wherein the status further includes calculations based on these relative 
volumes. 
According to still another aspect of the invention digital signal 
processing (DSP) means are provided for the calculations. 
According to a final aspect of the invention the sonic means further 
includes free-floating means, self orienting means, and tethered means, 
and the means for displaying and means for inputting are implemented by 
computer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention consists of a hermetically sealed self-orienting sonar sensor 
assembly that is joined by a wiring harness to an electronic signal 
interpretation module located at a convenient indoor location. The sonar 
sensing assembly floats near the top of the upper-most layer of material 
in a septic tank. It fires a sonar beam straight down towards the bottom 
of the tank. The resulting signal is processed by the signal 
interpretation module to yield the elevation of the top of the various 
layers. This information, in turn, is used to assess the volumes of the 
different components. This volumetric assessment provides an on-going 
measure of the remaining capacity of the tank for undissolved solids. 
These elevations are also used to assess whether either the top of the 
sedimentation layer or the bottom of the floating cake layer is 
approaching the bottom of either the output or the input baffles. If 
either of these conditions is true, then there is an emergency need pump 
the tank. 
The operation of the baffles can be understood by referring to FIGS. 1 and 
5, which depict an elevation view and a plan view of the septic tank. 
In the following section, the reference numbers in the following table may 
be of help in interpreting the operation of the preferred embodiments. 
______________________________________ 
element description 
______________________________________ 
1 Septic Tank casing 
2 access port 
3 inlet 
4 outlet 
5 inlet baffle 
6 outlet baffle 
7 liquid layer 
8 sedimentation layer 
9 cake layer 
10 in-tank unit 
11 Signal Interpretation module 
12 cable 
13 active sonar unit 
14 signal conditioner 
16 sound waves 
17 cable seal 
18 sensor orientation weights 
22 (DSP) module 
25 input/output (IO) panel 
26 keypad 
27 alpha-numeric display 
28 LED indicators 
29 audio alarm 
30 interface port 
31 signal interpretation module 
enclosure 
32 AC power cable 
34 central processing unit (CPU) 
36 power supply 
40 START 
41 sense keyboard 
42 Any User Input? 
43 Keyboard Input 
44 User Input Processing 
45 Stored Tank Parameters 
46 Sensor Input 
47 Calculate Interface Distances 
48 Update Tank Outlet Elevation 
49 Initial Filling? 
50 Sensor Elevation Stable? 
51 Evaluate Tank Capacity Metrics 
52 Pumping Required? 
53 Calculate Warning Level 
54 Present Pump Warning 
55 Update Percent Full Display 
______________________________________ 
The present invention, which relates to an improved septic system, may be 
understood by first reviewing the operation of a conventional septic 
system. Referring first to FIG. 1, which depicts a septic tank 1 of 
conventional size and shape, the septic tank itself provides an access 
port 2 used for pumping and other maintenance, and located at the upper 
part of the tank. The conventional septic tank also has an inlet 3 and an 
outlet 4. The inlet is conventionally protected by an inlet baffle 5, 
while the outlet is likewise protected by an outlet baffle. The baffles 
serve to isolate the inlets and outlets from the top and bottom layers of 
material that form during the normal course of operation of the septic 
tank. A plan view of the septic system, showing the baffles, is also shown 
in FIG. 5. Some tanks use alternatives to baffles such as "tee" pipe 
connections which perform the same function as the baffles, keeping the 
sediment and cake from flowing out of the septic system while the system 
still maintains system capacity. The current invention is equally 
applicable in the case of these alternative designs. 
FIG. 1 also shows the formation of the three layers typical of conventional 
septic tanks: the liquid layer 7, the "scum", or "cake" layer 9, and the 
sedimentation or sludge layer 8. 
During the normal course of operation of the convention septic tank, waste 
water enters the tank at inlet 3 and remains in the tank for a period of 
time that is a direct function of the size of the liquid layer 7 and the 
waste stream input flow rate. During this period of time, known as the 
retention time, suspended solids in the liquid layer migrate towards the 
top and the bottom of the tank resulting in the formation of the 
sedimentation layer 8 and a floating cake layer 9. Over time the thickness 
of both the bottom and the top layers increases. This, in turn, decreases 
thickness of the middle layer and thereby reduces the retention time. The 
reduction of retention time, in turn, reduces the settlement efficiency of 
the tank. When the middle layer 7 constitutes about 1/3 of the volume of 
material contained in the tank, it is common practice to recommend that 
the septic tank be pumped to remove all undissolved solids. If the tank is 
not pumped, eventually the middle layer will be reduced to such an extent 
that either 1) the inlet is clogged resulting in a backup of sanitary 
waste such that toilets and other waste sources no longer function or 2) 
suspended solids exit the septic tank into the drainage field in volume. 
These events typically do not occur simultaneously; the most common 
failure mode is the invisible one, where solids begin to enter the 
drainage field. Either condition is extremely serious and calls for 
immediate action on the part of the system operator. In most cases the 
primary component of the tank will be the lower sedimentation layer 8 at 
the time the tank needs to be pumped. In rare cases, the volume of the 
cake will dominate. 
The present invention includes a transponder contained within the septic 
system to detect the extent of the various layers, and to transmit related 
information to a remote computation and display system external to the 
septic tank. 
In the preferred embodiment of the invention, the in-tank unit 10, 
containing a sonar transponder--floats at the top of the uppermost layer 
of material in the tank. This transponder within this unit periodically 
emits a sound pulse directed downward. It is well known that changes in 
fluid density will cause such sound waves to reflect back toward their 
source. Thus the sound energy is reflected, to varying degrees, by each of 
the layers of material below the sensor. The time delay experienced by a 
sound wave is a function of the distance traveled, as well as the speed of 
sound of the wave in the particular medium. As a result, the transponder 
may calculate the distance to each layer a function of the time delay of 
the returning sound waves. The waves are generally encoded in pulses to 
facilitate time delay measurements. This technology, first developed in 
conjunction with submarine anti-warfare technology, has been developed to 
the point where it is used in many common domestic applications, such as 
fish finders used by recreational and commercial fishermen. 
The reflected sound energy is detected by the transponder and is then 
converted into electrical signals which are sent back to a signal 
interpretation module 11 via cable 12. 
Referring now to FIG. 2, the preferred embodiment of the in-tank unit 10 is 
shown. This external enclosure of this unit is constructed of high-impact 
plastic and is hermetically sealed to withstand the environment within the 
septic tank. The geometric shape of in-tank unit 10 in conjunction with 
the low center of mass guaranteed by the placement of weights 18 near the 
bottom of the unit guarantees a single stable orientation for the unit. 
The transponder in the preferred embodiment is of the type which generates 
a directional sound wave, and the sound waves in the current invention are 
directed downward toward the bottom of the tank. The preferred embodiment 
of the in-tank unit includes a commercial sonar sensor transponder 13 and 
an electronic component package 14 to condition the signal for 
transmission over a distance of up to 100 meters. The sonar transponder 
both transmits the sound waves and detects their return. 
The sensor assembly is connected to the signal interpretation module 11 via 
cable 12. This cable is designed to allow for direct burial. In addition, 
the portion of the cable that is within the septic tank itself is 
reinforced to allow it safely support the weight of the sensor assembly. 
FIG. 3 depicts the signal interpretation module, which is located in a 
convenient indoor location, often at a considerable distance from the 
septic tank itself. All of the signal interpretation components are housed 
with a single enclosure 31 constructed of a suitable material such as 
sheet metal or plastic. The signal interpretation module consists of a 
digital signal processing (DSP) module 22 which evaluates "raw" signals 
returned from the sonar transponder and converts these signals to a series 
of distance measurements from the transponder. Signals enter the DSP 
module via the cable 12 that is connected to the in-tank unit. The DSP 
module presents digital signals to a general purpose, low-cost, central 
processing unit (CPU) 34. The CPU interprets the distance measurements to 
determine the volumes of the tank components and for evaluating the 
location of the bottom of the top layer and the top of the bottom layer 
relative to the septic tank baffles. The CPU is also responsible for 
controlling the user input/output (IO) panel 25. The preferred embodiment 
of the invention includes a keypad 26 for entering key parameters 
including, but not necessarily limited to: 1) height of the bottom of the 
inlet baffle, 2) height of the bottom of the outlet baffle, 2) capacity 
percentage--the thickness of the central material layer in the tank as a 
percentage of the total depth where the tank is to be considered at its 
carrying capacity for undissolved solids. The preferred embodiment also 
includes an alpha-numeric display 27 that provides continual feedback as 
to "percentage full" and optional feedback predicting days remaining until 
pumping is required. The preferred embodiment also includes a set of three 
color-coded light emitting diode indicators 28 that indicate 1) no action 
is required, 2) pumping should be scheduled soon, 3) immediate pumping is 
required. In addition, the preferred embodiment includes an audio alarm 29 
to signal both the level (2) alter and the level (3) alert indicated 
above. The audio alert is accompanied by a "silence" button to turn the 
alarm off (not shown). The preferred embodiment of the invention also 
includes an interface port 30 for connecting the system to external 
monitoring equipment such as personal computers (PC's), home alarm systems 
and "smart building" systems. The preferred embodiment of the invention 
includes an integrated power supply of conventional design 36 suitable for 
powering the electronic components and the sonar transponder. An 
alternative embodiment incorporates the use of power supply external to 
the signal interpretation module. An additional alternative embodiment 
merges the DSP 22 and the CPU 34 modules into a single integrated module. 
FIG. 4 depicts a flowchart for the control software which resides in the 
Signal Interpretation Module. The software will run in a monitor loop 
characterized by the return to state 41 in the flowchart after each 
significant operation. A decision is then made based on whether any user 
keypad input is pending. If such input is pending process the input and 
store the results internally. If no such input is pending then control 
proceeds to read the sensor input 46. If user input is pending, then 
stored tank parameters are input 45. 
If no user input is sensed, control proceeds to the Calculate Interface 
Distances operation 47, which calculates these distances based on the 
digitally signal processed inputs of the DSP module 22. The outcome of 
this operation is a series of distance measurements from the sensor head 
to the various interfaces below it including the top of the sludge and the 
bottom of the tank. Control next passes to The Initial Filling decision 
49, which is based on whether the elevation of the sensor head has stopped 
increasing rapidly indicating that gray water has started to flow out of 
the tank. Initial filling indicates that either the tank has never been 
used before, or that it has just been pumped out. When the initial outflow 
of gray water is first detected, the elevation of the sensor is stored for 
future reference. If the system is not in the initial filling state, then 
control proceeds to evaluate the tank capacity metrics 51, and to update 
the display 55 to reflect the tanks current percentage full. (The tank is 
said to be full when it has reached its system capacity for undissolved 
solids, indicating pumping is required.) 
The tank capacity metrics evaluation consists of determining the distances 
between the various layer interfaces: the sediment, gray water, and cake. 
The distances to the tank bottom, and the levels of the inlet and outlet 
are also taken into account. 
After updating the display, a decision is made as to whether the operator 
should be warned of an imminent need to pump the tank 52, or if there is 
not an immediate need to pump the tank, whether an alternative level of 
warning should nevertheless be issued. If not, then the system returns to 
the keyboard sense state 41. Otherwise, the warning level is calculated 
53, and the present pump warning 54 is activated, illuminating one of the 
LEDs 28 on the control panel 31, shown in FIG. 3. 
If the system was in the initial filling state 49, then the elevation 
stability of the sensor is checked 50, and if unstable, the system returns 
to test the keyboard input state 41. In the case of stable sensor 
elevation, the tank outlet elevation is updated 48, the stored tank 
parameters are updated 45, and control proceeds to module 51, Evaluate 
Tank Capacity Metrics. 
The preferred embodiment of the control algorithms for the invention will 
allow the invention to be used effectively without requiring any 
configuration on the part of the septic system operator. If the operator 
elects to configure the system by entering parameters such as baffle 
heights, then the invention will be able to provide an improved level of 
emergency warning. The control algorithm will be such that temporary 
variations in elevations are ignored. The display will be based on a 
running average captured over a several hour period. 
The primary intent of this invention is to facilitate the monitoring amount 
of undissolved solid material in the tank; however it is also expected 
that this invention could be used to detect undesirable changes in the 
nature of the biological activity in the tank. Undesirable activity, as 
characterized by the death of waste digesting microorganisms, would be 
signaled by an increase in the rate of material deposition. This increased 
rate could be easily detected by the control software. It is also foreseen 
that the invention may be able to directly monitor the amount of suspended 
solids in the liquid layer by measuring the speed of sound waves through 
this layer. Direct measurement of the suspended solids would be 
facilitated by the addition of a temperature sensor to the sensor assembly 
10. 
In alternative embodiments of the invention the sensor assembly floats at 
the interface between the cake 9 and the fluid layer 7, as seen in FIG. 1. 
This alternative embodiment would allow for an estimate the thickness of 
the cake based on the known outlet height. In alternative embodiments the 
sonar sensor is attached directly to the bottom of the access port in 
conjunction with sufficiently powerful sensing technologies (i.e. both the 
transponder and the signal processing) employed. In the preferred 
embodiment of the invention the cable will enter the tank via the access 
port used for pumping. In some cases, field modifications to the access 
port may be required to enable the wiring to pass into the tank once the 
access port in place. In alternative embodiments, the sonar sensor is 
tethered so that it is located at a fixed location within the liquid 
layer. This particular alternative embodiment would involve two sonar 
transducers: one directed upward, and the other downward. Other 
anticipated embodiments of the invention may provide alternative means of 
routing the cable into the tank. 
The preferred embodiment of this invention incorporates a cable to connect 
the sensor in the tank to the signal interpretation module. Alternative 
implementations might include placing the signal interpretation module in 
an outdoor and environmentally hardened enclosure in the immediate 
vicinity of the septic tank, or using radio frequency (RF) communications 
so as to avoid the need to bury a cable between the signal interpretation 
module and the tank. 
Alternative embodiments may provide for either fewer or additional 
electronic elements in the sensor assembly itself. 
The primary intent of this invention is to facilitate the monitoring amount 
of undissolved solid material in the tank; however we also anticipate that 
this invention could be used to detect undesirable changes in the nature 
of the biological activity in the tank. Undesirable activity, as 
characterized by the death of waste digesting microorganisms, would be 
signaled by an increase in the rate of material deposition. This increased 
rate could be easily detected by the control software. 
It will be apparent that improvements and modifications may be made within 
the purview of the invention without departing from the scope of the 
invention defined in the appended claims.