If you live far from the main road…no sewers…option 1

What if you live too far from an STP?

A home septic System has similar processes

“Primary” … physical processes: settling and floating

Imhoff Tank

Primary Settling Tank (Imhoff Tank)

After the tank, “Secondary Treatment”

Junction box and leaching fields

Drinking water wells are best drilled uphill from Septic Systems

The Imhoff tank was developed to correct the two main defects of the septic tank.

It prevents the solids once removed from the sewage from again being mixed with it, but still provides for the decomposition of these solids in the same unit

It provides an effluent amenable to further treatment.

Contact between the waste stream and the anaerobic digesting sludge is practically eliminated and the holding period in primary settling compartment at the tank is reduced. The Imhoff tank may be either circular or rectangular and is divided into three compartments:

the upper section or sedimentation compartment the lower section known as the digestion compartment and the gas vent and scum section

• The Imhoff tank has no mechanical parts and is relatively easy and economical to operate.

• It provides sedimentation and sludge digestion in one unit and should produce a satisfactory primary effluent with a suspended solids removal of 40 to 60 percent and a BOD reduction of 15 to 35 percent.

• The two-story design requires a deep over-all tank. Primary tanks with separate digesters have largely replaced the Imhoff tank for large municipal installations.

• The Imhoff tanks is best suited to small municipalities and large institutions where the tributary population is 5,000 or less, and a greater degree of treatment is not needed.

• It is desirable to be able to reverse the direction of flow to prevent excessive deposition of solids at one end of the sedimentation compartment.

• Periodically reversing the flow will result in an even accumulation of sludge across the bottom of the tank. In operation, all of the wastewater flows through the upper compartment.

• Solids settle to the bottom of this sloped compartment, slide down and pass through an opening or slot to the digestion compartment.

• One of the bottom slopes extends at least six inches beyond the slot.

• This forms a trap to prevent gas or digesting sludge particles in the lower section from entering the waste stream in the upper section. The gas and any rising sludge particles are diverted to the gas vent and scum section.

Imhoff Tank Operation• There are no mechanical parts in an Imhoff tank. Attention

should, however, be given to the following:• Daily removal of grease, scum and floating solids from the

sedimentation compartment.• Weekly scraping of the sides and sloping bottoms of the

sedimentation compartment by a rubber squeegee to remove adhering solids which may decompose.

• Weekly cleaning the slot at the bottom of the sedimentation compartment. This can be done by use of a chain drag.

• Periodic reversal of flow where provided for in the design of the tank.

• Control of the scum in the scum chamber, by breaking it up, hosing with water under pressure, keeping it wet with supernatant from the digestion compartment and removal if the depth approaches two to three feet.

• Removal of sludge should be done before the sludge depth approaches within 18 inches of the slot in the sedimentation compartment.

• It is better to remove small amounts frequently than large amounts at long intervals.

• Sludge should be removed at a slow regular rate to avoid coning (i.e. the formation of a channel through the sludge) which would permit partially digested sludge and liquid held in storage above the digested sludge to be withdrawn from the tank.

• Before winter temperatures are expected, most of the digested sludge except that necessary for seeding (about 20 percent) should be removed to provide space for winter accumulations when digestion is very slow.

• The height of the sludge in the sludge compartment should be determined at inlet and outlet end of the tank at least once a month.

• After each time that sludge is removed, the sludge pipes should be flushed and drained to prevent sludge from hardening in and clogging the pipes.

• Prevention of "Foaming". Every effort should be made to prevent "foaming" because correction after the condition arises is sometimes difficult. "Foaming" is usually associated with an acid condition of the sludge and in such cases may be prevented or corrected by treatment with lime or sodium bicarbonate to counteract the acidity of the sludge. There are a few simple measures which may, under certain circumstances, remedy or improve the condition.

– The use of hydrated lime or sodium bicarbonate added to the gas vents will usually aid in correction. The pH value of the resulting sludge and lime mixture in the digestion compartment should not exceed 7.6.

– Removing the tank from service where possible for a few days and allowing it to rest will sometimes improve conditions.

– Agitation of the gas vent area with a water hose or paddles will sometimes help.

Working Principle

Settling of solids occurs in the upper compartment. Sludge falls through the slot to the bottom of the settling compartment into the lower tank, where it is digested.

Capacity/AdequacyImhoff tanks are used by small communities for primary treatment of grey- and blackwater.

Performance Removes 25 to 50% of COD. Pathogen removal is low.

CostsConstruction costs are slightly higher than the costs of a septic tank.

Self-help CompatibilityRequires expert design, but can be constructed with locally available material.

O&M

Should be checked for water tightness, scum and sludgelevels regularly. Sludge needs to be dug out every 1 to 5 years and discharged properly (e.g. in composting or drying bed). Needs to be vented.

ReliabilityReliable if amply designed and desludging carried out routinely. Imhoff tanks are resistant against shock loads.

Main strength Simple to construct and to operate.

Main weakness Effluent and sludge require further treatment.

• Advantages

• Solid-liquid separation and sludge stabilisationare combined in one single unit

• Resistant against organic shock loads

• Small space requirements

• The effluent remains fresh (i.e., not septic)

• Moderate capital costs, low operating costs

• Suitable for small settlements and house clusters

• Standardised designs available

• Simple operation and maintenance

• Disadvantages

• Very high (or deep) infrastructure; depth may be a problem in case of high groundwater table

• Requires expert design and construction

• Low reduction of pathogens

• Requires desludging

• Inefficient treatment option if not regularly desludged

• Odour occurs from escaping gases

• Further effluent and sludge treatment required

• Less simple than septic tank

Septic Tanks• The septic tank was one of the earliest treatment devices

developed. Currently, septic tanks provide wastewater treatmentfor small populations, such as individual residences, smallinstitutions, schools, etc.

• They are designed to hold wastewater at low velocity, underanaerobic conditions for minimum detention time of 36hours. During this period, a high removal of settleable solids isachieved.

• These solids decompose in the bottom of the tank with theformation of gas which, entrained in the solids, causes them to risethrough the wastewater to the surface and lie as a scum layer untilthe gas has escaped, after which the solids settle again. Thiscontinual flotation and resettling of solids carries some of them ina current toward the outlet to be discharged with theeffluent. The final effluent disposal occurs by subsurfacemethods. The effectiveness of this method is dependent on theleaching ability of the soil.

• These primary type units require a minimum of attention whichinvolves an annual inspection and the periodic (3 - 5 years)removal of sludge and scum accumulations.

• Septic tanks can be installed in every type of climate, although the efficiency will be affected in colder climates .Septic tanks are used for wastewater with a high percentage of settleable solids, typically for effluent from domestic sources .

• They can be introduced as a decentralized, on-site treatment system at household, block or school level.

• Effluents still contain pathogens and should therefore not be used for crop irrigation nor should it be discharged to canals or surface water drains. Effluents form septic tanks can be soil infiltrated in [soak pits].

• In more dense areas, the effluents should not be infiltrated but the septic tank may be integrated as individual pre-treatment units for a community into a small bore sewer system transporting the wastewaters to a secondary treatment. Even though the septic tank is watertight, it should not be constructed in areas with high groundwater tables or where there is frequent.

• Aqua privies can be built indoors and above ground and are appropriate for rocky or flood prone areas where pits or other technologies would not be appropriate, but they require frequent emptying and constant maintenance

• In more dense areas, the effluents should not be infiltrated but the septic tank may be integrated as individual pre-treatment units for a community into a small bore sewer system transporting the wastewaters to a secondary treatment. Even though the septic tank is watertight, it should not be constructed in areas with high groundwater tables or where there is frequent

• Aqua privies can be built indoors and above ground and are appropriate for rocky or flood prone areas where pits or other technologies would not be appropriate, but they require frequent emptying and constant maintenance.

Septic Tanks

Working PrincipleBasically a sedimentation tank (physical treatment) in which settled sludge is stabilised by anaerobic digestion (biological treatment). Dissolved and suspended matter leaves the tank more or less untreated.

Capacity/Adequacy

Household and community level; Primary treatment for domestic grey- and blackwater. Depending on the following treatment, septic tanks can also be used for industrial wastewater. Not adapted for areas with high groundwater table or prone to flooding.

PerformanceBOD: 30 to 50%; TSS: 40 to 60 %; E. coli: 1 log unitsHRT: about 1 day

Costs Low-cost, depending on availability of materials and frequency of de-sludging.

Self-help Compatibility Requires expert design, but can be constructed with locally available material.

O&MShould be checked for water tightness, scum and sludge levels regularly. Sludge needs to be dug out every 1 to 5 years and discharged properly (e.g. in composting or drying bed). Needs to be vented.

ReliabilityWhen not regularly emptied, wastewater flows through without being treated. Generally good resistance to shock loading.

Main strength Simple to construct and to operate.

Main weakness Effluent and sludge require further treatment. Long start-up phase.

Aqua privy

• The aqua privy is a variation of the septic tankand consists of a simple storage and settlingtank immediately under the latrine floor.

• Excreta drop directly into the tank through apipe. The bottom of the pipe is submerged in aliquid in the tank, forming a water seal toprevent escape of flies, mosquitoes and smell.The tank functions like a septic tank.

• The effluent usually infiltrates into the groundthrough a soak pit and accumulated solids(sludge) must be removed frequently. In anycase, the accumulating sludge must betreated.

• Advantages• Can be built and repaired with locally available

materials• No real problems with flies or odours if used

correctly• Long service life• Little space required due to underground

construction • Low investment costs, low operation and

maintenance costs depending on the availability of water and the requirement for emptying

• No energy required

• Disadvantages• High cost compared to dry or composting toilet systems• Constant and sufficient amounts of piped water

required to bring the waste to the treatment unit• Low reduction in pathogens, solids and organics:

Secondary treatment for both effluent and faecal sludge required

• De-sludging required: Manual de-sludging is hazardous to health and mechanical de-sludging (vacuum trucks) requires the infrastructure and may be rather costly

• Only suitable for low-density housing in areas with low water table and not prone to flooding

• Manual cleaning of the tank is highly hazardous and an inhumane task, while mechanical cleansing (vacuum trucks) requires sophisticated instruments