Small-scale Wastewater Purification

A. Plant systems

Method diagram

 

 

Method and installation description

Natural water purification relies on the self-purification processes of water, soil and plants. This wastewater purification technique uses no extra artificial energy supply or other necessary substances. Classic natural water purification is divided into purification in macrophyte beds, on the one hand, and helophyte filters on the other hand.

 

Macrophyte beds use water plants (e.g. water hyacinth, duckweed, etc.) for purification. Because this normally involves the use of floating plants, shallow basins are used.  The beds are often compartmentalised to prevent the wind from blowing away the plants. These systems are very sensitive to seasonal changes, whereby they tend to be implemented in warmer countries.

 

Helophytes are plants that are rooted in the soil, but with a relatively large surface area above the water surface. Tule, bulrush and reed are the most commonly used plants for these filters. Reed is preferred due to its favourable water purification properties. Thanks to its extensive network of roots and large quantity of biomass, reed has a large living surface for bacteria and other micro-organisms. These are responsible for a sizeable part of the purification effect. There are three types of helophyte filters:

 

  • Sewage fields: Helophyte filters with surface flow;
  • Root-zone fields: Helophyte filters with underflow;
  • Percolation fields or infiltration fields: Helophyte filters with vertical flows.

The biobed is a special type of helophyte filter. In this case, the plants are not placed in sand, but in various layers of coconut material. This material retains the wastewater but, simultaneously, has enough pores to introduce oxygen into the system. Just like the reed field, purification takes place via filtration, intake of nutrients by plants and biological activity.

 

In practice, it is best to deploy pre-sedimentation (e.g. septic pit) in order to remove suspended matter. This prevents clustering and potential system blockages.

 

Specific advantages and disadvantages

Due to the low energy consumption (pumping) and the natural appearance of the system, the helophyte filter is an appealing technique for small-scale water purification. The system requires little maintenance or follow-up. There is no sludge production.

 

One of the disadvantages of this system is the long start phase needed to form a suitable root system. Further, the free space needed is relatively large (up to 5 m²/IE[1] for a root-zone reed field) and the purification effect is determined by the seasons.

 

Application

Sectors where helophyte filters are implemented include slaughterhouses, frozen food companies, dairy companies, jam-making companies, potato-processing and vegetable-processing companies, tobacco companies, etc.

 

Systems are placed as primary, secondary and tertiary purification systems.

 

Boundary conditions

The wastewater is often subject to purification prior to the plant system. This consists of a septic tank and possibly a fat/oil separator.

 

 A helophyte filter is primarily intended for the removal of suspended matter (SM) and organic load (COD and BOD). Systems are dimensioned depending on the type of application. A system for tertiary purification will, for example, be dimensioned for far-reaching removal of SM, residual BOD and P.

 

Effectiveness

Plant systems can be implemented for the removal of the following parameters (among others): COD (85-90%), BOD (90-95%), SM (70-90%), N (>50%) and P (25-65%). The purification yield is greatly determined by the dimensioning (size of load).

 

The purification efficiency is season-related. The highest bacterial and vegetative activity is found in summer and autumn. During winter and spring, the yield can drop by 40-50%. This is not always noticeable in every plant system.

 

Support aids

In certain cases, lime, Fe or Al shavings are added to remove extra phosphorous.

 

Environmental issues

/

 

Costs

Case 1:

 

The investment cost for a percolation field with a surface area of 240 m² (ca. 80 IE) is estimated at € 27.500.

 

Case 2:

 

The estimated investment cost for a percolation field for the purification of 175 IE, amounts to € 80.000 (installation included).

 

 Operating costs per year:

 

  • Energy consumption (1 €/IE.yr): 175 €;
  • Personnel costs (0.5 h/wk x 220 d/year x 25 €/h): 590 €;
  • Maintenance (0.5 % of investment): 400 €.

Comments

The plant system must be subject to thorough maintenance once a year.

 

Complexity

Not applicable.

 

Level of automation

Not applicable.

 

References

  • Derden A., Meynaerts E., Vercaemst P. and Vrancken K, Best Available Technique (BAT) for cattle sector, Academia Press, ISBN 90 382 0945 2, xiv + 289 pp., 2006
  • EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
  • VITO-SCT, revision of technical files WASS, 2009

 

 

B. Septic Tank

Method diagram

 

 

Figure: cross-section septic tank.

 

Method and installation description

The septic tank is a wastewater purification technique that dates back to 1860. Purification is based on three processes: separation of sedimentary matter, separation of suspended matter (e.g. oils) and anaerobic degradation of organic pollutants.  The level of purification is influenced by the volume of the tank. This determines the retention time, amount of dilution and shock resistance.

 

Septic tanks are pre-fabricated and are placed underground. Concrete and plastic are commonly used materials. The structure of a septic tank can vary from 1 to 3 compartments and between with or without air ventilation and air treatment. Most sedimentary matter is separated in the first compartment, where the sediment slowly forms a layer at the bottom. This is primarily organic matter, and is subject to anaerobic fermentation resulting in (among others) the formation of CO2, CH4 and H2S. The formation of hydrogen sulphide gas is normally prevented because sulphide precipitates with the present metals. Fats and floating substances are also separated, while the wastewater flows through to a possible second compartment. The sedimentation and floating process then repeats itself.

 

A septic tank must be emptied on a regular basis. A limited layer of sludge must always remain present as a source of bacteria. The removed mass is normally discharged into a sewer-water purification system.

 

Specific advantages and disadvantages

Septic tanks are very easy to implement, require little maintenance and are cheap to operate. The theory is primarily based on the separation of sedimentary and suspended matter. A septic tank has a limited purifying capacity (25-45% for BOD and 20-90% for SM). Due to the limited purification capacity, it is normally not possible to discharge the wastewater after a septic tank; thus an extra treatment is needed.

 

Application

Septic tanks are often implemented in remote locations where there are no sewers, such as agricultural businesses, restaurants and camp sites.

 

Boundary conditions

The size of a septic tank is based on a retention time of ca. 3 days.

 

Because fermentation is a biological process, biologically degradable cleaning products are preferred. Toxic products may disrupt the biological equilibrium in the tank.

 

A septic tank must be cleared on a regular basis (annually). Each time the septic tank is emptied and cleaned, it must be completely filled with water before it is put into use once again. 

 

Effectiveness

A septic tank primarily removes sedimentary and floating matter. The yield for suspended matter amounts to 20-90% (depending on correct dimensioning). For organic dirt load, a reduction between 25-45% can be realised. The septic tank is normally followed by a biological treatment.

 

Support aids

None

 

Environmental issues

Stabilised sludge is released as by-product. Additional measures may be necessary to address odour problems.

 

Costs

The following table provides a few guide prices for various septic tanks, without pumps, installation and start-up.

Capacity (L)

 

Price (€, incl. VAT)

 

1500

 

195

 

3000

 

290

 

7500

 

730

 

10000

 

850

 

Comments

None

 

Complexity

Not applicable.

 

Level of automation

Not applicable.

 

References

  • TNAV, supplier survey, 2008
  • VITO-SCT, revision of technical notes WASS, 2009
  • www.vlario.be

 

 

Version February 2010

 


 

[1] RE: Resident’s equivalent or a measure of wastewater pollution 1 IE is consistent with a theoretical volume of 150 litres per day and a daily load of 90 g Suspended Matter (SM), 135 g Chemical Oxygen Demand (COD), 60g Biological Oxygen Demand (BOD), 10 g nitrogen (N) and 2 g phosphorus (P)