Prefermentation of Primary Sludge

13-jun-2006, STOWA





South Africa

Stage of development

Full scale

Process  -





Hydrolysis  -  Enhanced nutrient removal



Primary sludge



Biological hydrolysis and acidogenesis

Keywords: decrease sludge production; decrease energy demand; sludge line




Prefermentation is the production of soluble organics such as volatile fatty acids by subjecting municipal wastewater to anaerobic conditions for a period of time. The full-scale prefermentation of primary sludge was implemented successfully at South African WWTP's in the early 1980s as a primary treatment process to enhance nutrient removal in activated sludge processes.

It is hereby explicitly acknowledged that the following text is based on publications by Rössle [2,3] and Münch [1]. Slightly changed wording of their texts, where appropriate and necessary, has led to the omission of quotation marks in the following text.


Description and working principle

Prefermentation is aimed to improve the performance of activated sludge processes with biological nutrient removal by providing easily degradable, soluble  organic matter under anaerobic conditions. The anaerobic digestion process, traditionally used to prepare sludges from wastewater treatment for ultimate disposal, has been extensively researched during the past decades. Prefermentation is the first phase of this digestion process, where mainly hydrolysis and acidogenesis take place.

Other anaerobic biodegradation processes, such as acetogenesis and methanogenesis which lead to prefermentation failure, are suppressed in practice by maintaining a low solids retention time, which ensures that methanogenic bacteria are washed out of the prefermenter.

The produced volatile fatty acids are consumed in a number of simultaneously occurring biological processes, such as aerobic respiration, sulphate reduction and denitrification. These processes have to be considered when evaluating prefermenter performance.

The main prefermenter process configurations are classified according to the number of tanks used, the flow configurations and sludge retention time control modes. An in-line prefermenter, fed with raw sewage, consists of a gravity thickener equipped with a settled sludge recirculation capacity. A side-stream prefermenter, fed with primary sludge from an upstream primary settling tank, consists of a mixing tank, a thickener or both. The most common prefermenter configurations are shown in the following table.

The major advantage of an in-line configuration is the lower capital construction cost and a compact civil design due to lower space requirements. The major advantage of a side-stream  configuration is the larger operational flexibility and control allowed, which can be even more beneficial in the absence of raw sewage hydraulic equalisation or flow balancing facilities at a WWTP. In a side-stream prefermenter the sludge blanket level control, and therefore, the sludge age in the secondary tank, are simply controlled by adjusting the waste sludge from the secondary tank, together with the prefermenter inlet primary sludge flow from the primary settling tank.

The dual physical and biological functions of settling and fermentation in a single activated primary tank (APT) require appropriate management strategies. The relationships between suspended solids and settleable solids concentrations, retention times, overflow rates, throughflow and settling velocities and tank shape which are found in primary settling tanks are complicated in an APT by the additional retention times and recycle rates required for prefermentation.


             Process configurations [3]


Inline and side-stream  fermenter

Most full-scale prefermenters are fed with sludge from primary clarifiers. These prefermenters are called 'side-stream prefermenters'. The so-called 'in-line prefermenters' are fed with raw sewage rather than with primary sludge.

A number of different prefermenter configurations have been proposed, each of which represents a compromise between reliability of VFA production and associated costs. The currently used prefermenter configurations can be classified according to the number of tanks needed for their realisation. The most commonly used prefermenter configurations are discussed in more detail below.

Text Box:  Activated primary tanks (APT)

APT's produce VFA's by allowing a sludge blanket to form in the primary clarifier. Fermentation and VFA production then take place in this sludge blanket. The sludge is recycled back to the inlet and this recycling process is supposed to inoculate the incoming solids with actively fermenting organisms, elutriate the VFA's formed in the sludge blanket.

Single-stage prefermenters

These prefermenters can be either mixed or non-mixed.

Non-mixed single stage prefermenters (often called 'static prefermenters') are basically an extension of the fermenting potential in the primary clarifier. The SRT is controlled by keeping the sludge blanket height constant.

Two-stage prefermenters

Pimary sludge is fed to a completely-mixed tank and subsequently to a thickener for removal of the VFA-rich supernatant and for control of the solids retention time.


Prefermenter configurations [1]            


Design guidelines / Technical data

A list of design and operational parameters as well monitoring guidelines for prefermenters was compiled by [3]. A table taken from this publication is listed at the end of this document under the heading Graphics.

It has been reported that general operational guidelines are still under development and it has been suggested that local conditions may strongly influence design and performance of prefermenters. More information on general design guidelines can be found in [3], where design parameters are dicussed in detail.

Prefermentation in primary clarifiers is achieved by raising the sludge blanket level. By recirculating sludge to the primary clarifiers the VFA are then elutriated. However, prefermentation in the primary clarifier increases the risk of sludge wash-out under wet weather conditions and can thus affect negatively the overall treatment process. Under the Dutch conditions with its high DWF/WWF ratios, prefermentation in primary clarifiers or other separate reactors in the main treatment train is not attractive.

Temperature and retention time are important factors in determining prefermentation efficiency. Some general guidelines are given in the following table. Research indicates that hydrolysis is most efficient at temperatures of 25 to 30°C and a retention time of 2 to 3 days.


Temperature [°C]

Retention time [h]


10 - 20

40 - 100


30 - 40

15 - 30


50 - 70

5 - 10

The production of VFA lowers the pH. At a pH of 5,3 to 6,0 the hydrolysis stops. Alkalinity determines the rate with which the pH decreases. With low alkalinity and high sludge concentrations the pH can limit the production of VFA.

By prefermentation about 7 to 8 % of biodegradable, dissolved COD can be made available for biological nutrient removal. This COD fraction is not available for digestion. This has to be taken into account when calculating digester gas and energy production.

Under Dutch circumstances prefermentation seems attractive in combination with preprecipitation. Regarding the generally low COD/N ratios in Dutch wastewater, prefermentation may also be combined with primary clarification.



The following table gives an overview over prefermenter functions and reported performances [based on 3].


Performance level

Removal of

settleable solids

~ 90 to 95%


suspended solids

~ 50 to 80%



~ 30 to 50%


TKN and TP

~ 15 to 25%


floatable material

Depending on local operation and facilities

Thickening of settled solids

Maximum of 6% TS, 1% TS required if additional thickeners available

Reduction in secondary (activated) sludge yield

Average 50 to 70%

Equalisation of the inflow

Depending on local operation and facilities

Rate of VFA production

Maximum 0.2 VFA/mg COD (side-stream) or 1 to 70 mg VFA/l.h

Elutriation of fermentation

Depending on local operation and facilities

Eliminate need for chemical precipitation of P

Lower sludge yield (~ 25%), salinity and alkalinity not influenced


By comparing the inflow, accumulated sludge and outflow VFA concentrations, the prefermentation efficiency and production of VFA in the prefermenter can be monitored. The production of VFA is supposedly the best measure of general prefermenter efficiency. The required production also determines the required design tank volume. For side-stream prefermenters values are ranging between 15 to 70 mg VFA/l.h, for APT values of less than 10 mg VFA/l.h are reported.


Operational stability and maintenance

The operational complexity of a prefermenter is strongly dependent on the process configuration. Operational parameters as given in the table on the next page are discussed in detail and for different process configurations by [3].

In general a prefermenter operational evaluation will be based on wastewater characteristics, as well as sludge blanket level and sludge wastage management for proper control of the solids retention time.

A proper operational strategy should minimise and justify the degree of odour generation, which causes resistance for prefermentation implementation, specifically in circumstances where uncovered prefermenters are used.


Capital and operating cost

The economic feasibility of the process under Dutch circumstances has been investigated by [].

Three main factors are influencing the economic feasibility:

·       hydrolysability of primary sludge: the amount of COD that can be made available by prefermentation will be strongly dependent on the type of sewerage system as well as seasonal influences

·       liquid/solids separation efficiency: the production of VFA will increase by using mechanical thickening

·       actual COD demand in the treatment process

In the table below the capital and operating cost for prefermentation with and without mechanical thickening are given and compared to an actetate dosing installation for a WWTP  with 100.000 p.e. cap. [4].


Investment cost (euro)





Acetate dosing


Operating cost (euro/y)




Capital cost








Energy demand:






Reactor mixing





Sludge heating




Decreased biogas/energy production




Total annual cost 148.950





Based on the cost given above and an estimated prefermentation efficiency the cost per kg CODproduced can be calculated, the results are given in the table below. The prefermentation efficiency is therefore the most important factor in establishing the economic viability of the prefermentation process. This needs special attention in evaluating and minimising operational risks and uncertainties [4].

Prefermentation eff.

[g Ac/kg dry solids]

VFA product.


Cost per kg dosed acetate

[euro/kg Ac]





Acetate dosing
















Reference installations

Two reference installations arereported by [5].


Suppliers / Patents

Extensive research into the application of prefermentation has been carried out at the University of Pretoria.

University of Pretoria

Dept. Chemical Engineering - Water Utilisation division

Pretoria  -  South Africa

Tel: +27 11 - 316 1304  -  Fax: +27 11 316 1928  -  E-mail:  -  Web:


Literature references

[1]   von Münch, E.: Prefermentation technology for BNR plants. Online publication on the website Activated Sludge Pages -

[2]   Rössle, W.H.;Pretorius, W.A.: A review of characterisation requirements for in-line prefermenters - Paper 1: Wastewater characterisation. Water SA, 27-3, July 2001.

[3]   Rössle, W.H.;Pretorius, W.A.: A review of characterisation requirements for in-line prefermenters - Paper 2: Process characterisation. Water SA, 27-3, July 2001.

[4]   Stowa report: Options in primary treatment. 96-20. 1996.

[5]   Communication with SSI, South Africa. 2002.





ADWF - average dry weather flow

APT - activated primary tank

COD - chemical oxygen demand

DR - downflow velocity rate (stilling chamber)

HRT - hydraulic retention time

HRT eff - effective hydraulic retention time

N - nitrogen

NH 3 - ammonia

NH3 +NH4-N - total ammonia nitr.

NO 3 - nitrate

o-PO 4 - orthophosphate

P - phosphorus

PST - primary settling tank

r VFA - rate of VFA production

SER - sludge elutriation rate

SetS - settleable solids

SRR - sludge recycle rate

SRT - sludge retention time

SS - suspended solids

T - ambient temperature

TKN - total Kjeldahl nitrogen

TOD - total oxygen demand

TS - total solids

UR - upflow velocity rate (tank)

VFA - volatile fatty acid


             Guidelines and design parameters as compiled by [3]