Throughout the last five years WPL has supplied Yorkshire Water Services (YWS)

with package plant and has built a close working relationship with the water

company during this time.

The study criteria focuses on six main areas of interest:

  • wastewater characteristics and flows,
  • operational problems,
  • control and maintenance,
  • operating costs and power consumption,
  • performance,
  • recovery from shutdown,

Tighter environmental regulations make it necessary for secondary treatment

of domestic wastewater at plants dealing with a population equivalent (PE) of

less than 2,000. As part of this programme Yorkshire Water Services has monitored

the performance of nine WPL SAFs with sizes between 20-250PE.

Small treatment systems are not a scale-down of larger systems. In contrast

to their larger counterparts, issues of short-circuiting, solids breakthrough

and prolonged periods of no flow with intermittent high peak flows have potential

to reduce performance and need to be addressed.

A package sewage treatment system should provide advanced treatment to meet

a range of different standards, accommodate large variations in hydraulic and

organic loadings, be simple and easy to operate, be cost effective and be regularly

operated and controlled by trained personnel.

Sites B, D, G and I were monitored intensively, with composite samples taken

of the raw sewage, settled sewage and effluent on a daily basis. Spot samples

were also taken from every other biozone compartment and flow levels were recorded.

Weekly samples were taken from the remaining five sites.

The four sites visited frequently were monitored for flow and BOD throughout

the day which gave a diurnal flow and load profile. The results indicated a

typical peak in flow during the morning with a lower peak in the evening. The

only exception was Site G which had a pub in the catchment area and here the

evening peak was higher. See Figure 1. The strength of BOD and ammonia in the

wastewater also showed similar peaks.

The smaller the plant, the more accentuated the peaks, in terms of maximum

peak to average flow ratios. However, despite the high peaks at Site B, performance

was reliable.

The wastewater analysed was typical medium strength domestic sewage (as defined

by Metcalf & Eddy), ie BOD at 185mg/l and ammonia at 39mg/l. In all cases

there was adequate pH and alkalinity to allow nitrification to proceed.

The wastewater characteristics showed sharp diurnal variations with concurrent

flow and strength, morning and evening peaks. There was no sign of a third early

afternoon peak as has sometimes been suggested.

The scale of variation in the flow of pollutants varied inversely with the

size of the community. Overall the SAFs coped well with the variations in loading,

as reflected in minimal diurnal final effluent variations. In all cases estimated

flows and loads were adequate to ensure sufficient plant design. Accuracy in

estimated flow data increased with larger plants. There was no indication that

oversized plants caused performance problems.


Some operational problems which affected performance were identified.

The study also surveyed operators and managers. The sites were reported as

“lovely” and “easy to operate”. Suggestions for improvement

included automatic restart. The tanker drivers were equally happy and reported

a de-sludging time of approximately 15 minutes. Little information could be

found with regard to real maintenance costs, partly due to lack of recording

and little requirement for maintenance work.

Small treatment plant failure is generally due to poor design for the conditions

and mechanical wear and tear. The SAF survey did not discover any failure modes

directly related to poor flow estimations and unsuitable design. Any problems

were linked to lack of operational control.

The visit regime for sites is less frequent than recommended for small-activated

sludge, trickling filters and rotating biological contactors (RBCs). The extended

interval can be justified on the basis that a SAF is considered more biologically

robust than the activated sludge and trickling filter processes, and more mechanically

reliable than the RBC process.

WPL’s design is generally accessible, with large lids enabling easy visual

inspection. However, an efficient operator will only spend 15 minutes on-site

per inspection, so it is felt the airlifts should be set with no more than a

10-minute off-time, so it can be checked during a routine visit.

Power usage

Figure 2 displays the power consumption per PE on a daily basis and indicates

that economies of scale take place even between plants ranging from 20-250 PE.

There is a clear per capita saving on power consumption as the plants increase

in size. It is evident that SAFs use more energy than RBCs. However, Fastenau

et al (1990) reported a breakdown frequency for RBCs in the range of one-three

times/pa. The savings in power consumption are, consequently, offset by the

cost of shaft breakdowns and replacements.


Figures 3 and 4 indicate that on average the plants operated within the descriptive

consent.. Most plants were capable of achieving it under normal operating circumstances.

Figure 5 shows the average performance of the intensively studied plants in

terms of suspended solids removal. The figures indicate the plants perform reliably

when their operation is not interrupted, for example, by a power cut.

The plants were not designed to nitrify but in most cases there was sufficient

capacity, due to design safety margins, which allowed some nitrification to

take place. In the case of the three plants with no performance interference,

there was sufficient capacity to nitrify to 10mg/l ammonia with a 0% chance

of a 95%ile failure, demonstrating the ability to nitrify was reliable.

Power cuts caused odorous sub-standard final effluents within 24h. Figure 6

demonstrates how the biology at Site G recovered after a five-day period without

aeration. It is clear the less sensitive carbonaceous oxidation process meets

the consent within 24h. Similarly, suspended solids decreased to below 30mg/l

within 48h. The ammoniacal nitrogen (ammN) removal had a much longer recovery

time. At Site G, a 14-day period was necessary for the ammN levels to fall below

10mg/l. At Site F the recovery period for ammN was nine days after a shut-down

period of eight days, and at Site E the recovery period was two days after a

shut-down period of three days. All plants mentioned had capacity to nitrify

to varying degrees.

In all cases plants performed within consent when no operational incidents

interfered. Biological treatment was the most reliable with the plants more

susceptible to solids breakthrough than BOD or ammonia. Despite the high peak

flows experienced by smaller plants, performance was maintained with low-zero

probability of breakthrough in BOD and ammonia.

Primary tanks performed as expected with 30-40% BOD reduction. Plants with

separate sludge storage had reduced efficiency, possibly due to the return of

particle rich liquors to the settlement tank.

Where existing septic tank primary tanks had been used, BOD reduction was comparable

to the WPL primary tanks, but de-sludge took longer. However, ammonia levels

tended to be higher from the septic tanks.

Biozones held dissolved oxygen (DO) levels above 2mg/l and pH was maintained

between six-nine. DO levels rose across the segmented biozone as BOD, ammonia

and alkalinity levels dropped. The final settlement tank removed between 60-99%

of suspended solids, although on some occasions soft flocs of biomass were noted

in the final effluent under normal operating conditions. Overall the SAFs accommodated

load variations well, as reflected in minimal diurnal final effluent variations.

The report identified the following advantages and disadvantages associated

with SAF technology:


  • no recirculation of flow or pumping of backwash water,
  • no moving mechanical parts,
  • little surplus sludge production due to high micro-organism bio-diversity

    and long sludge retention time,

  • the process can sustain and adapt to fluctuations of hydraulic and organic

    loading since it possesses a larger amount of biomass and a longer food chain

    compared to activated sludge,

  • the long retention of biomass solids gives good protection against toxic


  • high surface-to-volume ratio of support media offers the possibility of

    a significant reduction in volume, compared to suspended-growth systems.


  • the process is not maintenance free,
  • small mechanical units are more susceptible to breakdown than large units,
  • high turbidity in final effluent can occur because of tiny particles of

    broken biomass that does not settle well,

  • medium-term extra cost,
  • poor sludge stabilisation compared to suspended growth processes,
  • slow start-up and time needed before full recovery after system breakdown,
  • little flexibility once designed,
  • some odour problems have been reported,
  • no correspondence in actual air requirement and air supply throughout different

    stages of the process,

  • accumulation of fats, oils and greases can cause clogging problems,
  • detergents can lead to foaming problems,
  • suspended growth systems have better oxygen transfer efficiency,
  • tank dimensions are limited by the need for transportation,
  • compactness can lead to more turbidity and higher temperature effects.

The report concluded:

  • in Yorkshire Water Services the estimated wastewater characteristics safety

    ensured the plants were reliably

    compliant to their discharge consent,

  • existing primary septic tanks could be used for BOD removal systems,
  • separate sludge storage tanks reduce the efficiency of the primary tank,
  • none of the evaluated plants were overloaded,
  • overall consent compliance tended to be compromised by operational or M&E

    problems, rather than improper process design,

  • the SAF plants evaluated had minimal maintenance costs,
  • new installations should be sited as far from local residents as possible,
  • frequency of maintenance, control and mechanical breakdown will affect

    the overall success of the SAF plants,

  • improvement recommendations to the SAF include auto-restart in working

    order and feed forward airlift should be operated at intervals of less than

    15 minutes,

  • a SAF is considered more biologically robust than the activated sludge

    and trickling filter processes, and more mechanically reliable than the rotating

    biologicalcontactor process.

Commenting on the study, Richard Munden, managing director of WPL, said:

“Being involved in the trial has been a very worthwhile experience. We’re

delighted with the results. Customers regularly advocate the design and reliability

of our plant and the trial has reinforced this message. With regard to operational

events and their implications on performance, this is an area, as a manufacturer,

over which we have no control. However, I am sure that the comments relating

to this issue will be extremely valuable to our customers.

We also read with interest the improvement recommendations and will seriously

consider them as part of our ongoing research and development programme. Customer

feedback is always welcome, as we strive to constantly improve our product range.”

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