What is the truth about land-based treatment, soil beds and infiltration?

Some extracts from the interesting report that you can download above.

BAGA Water Technology AB has followed the debate regarding the function, lifetime and ability of soil beds to reduce phosphorus and nitrogen with some amazement. Sensitivity to sludge escape, the need for ventilation, the cause of clogging, etc.
The debate and discussion have arisen because there are some poorly functioning plants, which is why various more or less serious explanations for the cause of the problems have seen the light of day.
Clogging of a soil bed or infiltration is almost always due to a poorly functioning sedimentation in the sludge separator in combination with poor oxygenation of the bed.

The sedimentation problems in the sludge separator can be explained by several reasons:

– The sludge separator is not sufficiently ventilated, so gas formation will cause large amounts of floating sludge and a sharp deterioration in the sedimentation capacity of the sludge separator.
– The backwash water of any water treatment equipment installed has been incorrectly connected to the property’s sewage system. This negatively affects the function of the sludge separator.
– Fleet oils, high levels of animal fats, yoghurt spills, etc. should not be discharged into the
sink as they clearly impair the function of the separator.
– The sludge separator has not been emptied in time, and sludge is carried out to the infiltration or soil bed.
– Excessive water flows through the sludge separator, which can stir up settled sludge and result in a large discharge of particles to the subsequent infiltration or soil bed.
Examples are:
– heavy surface water (rainwater) infiltration
– when water is discharged from a spa

In summary, the following implies for the selection of a sludge separator:
The ability of the sludge separator to separate sludge is essential for the discharge quality of the wastewater and has a decisive impact on the lifetime of the plant. A well-functioning sludge separator reduces the biological load on the subsequent infiltration or soil bed.

The sludge separator must be properly ventilated through the property’s piping system up over the roof ridge. If the wastewater is pumped to the sludge separator, another ventilation system must be installed to ensure ventilation.

Never load a sludge separator with higher flows than it is designed for. A certified sludge separator for 1 household is not intended to be loaded with more than a maximum of 1,000 liters per day distributed throughout the day with a maximum of 200 liters per hour, regardless of the fact that tests have been carried out with peak flows of 30 liters per minute for 10 minutes. If there is a risk of higher flows,
a larger sludge separator with better flow performance should be selected.

As for the construction of the subsequent infiltration or soil bed, the technology has evolved over the last 10 years. Most of the necessary washed sand and macadam has been replaced by different types of carrier materials for the biofilm
(bacterial culture). The size and thickness of the beds have been reduced from 30-40 m2 to 15-20 m2 surface, while the height of the sand and macadam layer has been reduced from 90-100 cm to 30-50 cm. A not insignificant reduction in sand and macadam. A reduction of 15-20 m3 of sand and macadam for a soil bed intended for 1 household. A large financial saving, including fewer transportation and working hours.

They have also become more efficient and safer. A properly constructed infiltration or soil bed has a lifespan of at least 20-30 years, provided that the sludge separator effectively prevents particulate pollutants from entering the subsequent infiltration or soil bed.
Infiltration systems no longer require the addition of sand or macadam masses, provided that BioModules are used in combination with spreader plates. BioModules with spreader plates ensure the functioning of the infiltration.
The main part of the biological process takes place in the BioModules, and the spreader plates guarantee the dispersion and oxygenation of the total infiltration surface.
Of course, the total infiltration surface must have been dimensioned according to a correctly performed percolation test, or sieve curve of the underlying soil masses.

Reduction of phosphorus
A land-based sewage treatment plant in a not densely populated area where the infiltration bed is constructed at least 100 meters from a lake, water-bearing ditch or drainage, rarely needs any additional treatment.
A well installed infiltration plant with bioModules and spreader plates will meet 90% reduction in BOD7 and with some doubt 50-70% reduction of Ptot depending on the soil conditions.

If the plant is to reliably achieve 70% reduction of Ptot, the incoming wastewater should be precipitated in the sludge separator. Flow proportional precipitation normally achieves up to 98% reduction of phosphorus over the bed.

It can never be guaranteed that a soil bed or reinforced infiltration achieves at least 70% reduction of phosphorus, i.e. normal protection level, without some form of supplementary treatment.

There have been a very large number of tests and analyses regarding the reduction of phosphorus in different types of beds. To claim that at least 70% phosphorus is reduced without the use of flocculants or any form of subsequent phosphorus trap seems more like a
frivolous sales argument. Of all the studies and analyses done, there is nothing to support that at least 70% reduction is achieved. Possibly the result can be achieved in a relatively newly built bed.
From BAGA we have followed 2 soil beds without installed precipitation, for more than 6 years.

The results for Ptot reduction are as follows for phosphorus emissions:
Age (years) Phosphorus mg/l
1 1.2
2 1.7
3 2.5
4 3.2
5 4.2
6 5.6

Both soil beds are constructed according to Fig. 3, but have a layer (thickness) of 90 cm of washed 0-8 sand. Our checks clearly show that a soil bed will not fulfill the requirement of 70% reduction of phosphorus, except for the first years.

Please read David Eveborn’s research report “Sustainable phosphorus removal in onsite wastewater treatment” presented May 2013 KTH. David Eveborn’s report shows that it is the chemical processes in the soil that determine how well or poorly soil-based wastewater facilities work. It is mainly aluminum and iron compounds in the soil that bind phosphorus and the retention works best in soils with
low pH. It has also been shown that these systems can leach out already bound phosphorus.

Precipitating the phosphorus and binding it in the sludge settling in the sludge separator has many advantages, but most importantly, the phosphorus can be recovered from the sludge.

It is highly questionable to use any form of phosphorus trap with reactive filter material after the sludge separator, as the treated water cannot be infiltrated. All microfauna will die due to the high pH value, between 10.0 and 12.0. The infiltration bed will also quickly become clogged.

When using a phosphorus trap after a soil bed, it must be ensured that the discharge does not affect plants and wildlife at the discharge point. Thus, a small water-bearing ditch is not a suitable recipient. In addition, installing a phosphorus trap after an older sewage system that may not function properly usually causes problems in terms of operation and maintenance.

An investigation carried out in 2015 by Charlotta Larsson in collaboration with the City of Trollhättan shows that most of the plants supplemented with phosphorus traps do not function satisfactorily. See report “Phosphorus traps in Trollhättan municipality”.
64 individual wastewater facilities with subsequent phosphorus traps were inspected. The result was devastating, 63% of the phosphorus traps were more or less affected by organic material from the previous treatment step. The most serious thing is that only four property owners out of 64 had changed the filter material at some point.
Perhaps not so surprising, as a change costs SEK 7-8,000 and must be done as soon as the pH of the outgoing water falls below 9.5 and the phosphorus trap then stops working. It is expensive and needs to be changed often.

Any flocculant application must be flow proportional. Applying flocculant in a timed and random manner does not work well. It is self-defeating because applying the flocculant disproportionately to the property’s sewage system gives an uncertain and poor result. The efficiency will be poor and the risk of crystallization causing deposits in the pipe system is obvious. When using flow-proportional addition of flocculant directly to the sludge separator, a final reduction of more than 90% in phosphorus is easily achieved, thus meeting the “high level”. As an added bonus, a higher
nitrogen reduction is also obtained in the subsequent soil bed due to the lower organic load of the bed, which is caused by the fact that the flow-proportional addition of flocculants also reduces BOD, COD and suspended solids by about 50-70%.

The high reduction of suspended solids is a prerequisite for high hygienization. Measurements carried out by JTI (RISE) after the beds have shown a water that is basically bacteria-free. Theoretically, this can be explained by the fact that 85-90% of the bacteria are precipitated in the sludge separator, and the remaining 10-15% disappear in the subsequent step. The flocculant technology also means that the pH at discharge to the receiving water is normally around 7.5-8.0.

Reduction of nitrogen
The reduction of nitrogen is always related to the structure, size and load of the bed.

The pre-treatment of the wastewater and the size of the bed are thus of the greatest importance for the total reduction. A prerequisite for high nitrogen reduction is that the soil bed or reinforced infiltration is low in BOD7 and CODcr This is achieved by the flow-proportional addition of flocculants directly to the sludge separator, and by
increasing the size of the bed. The reduction of the bed load in terms of BOD and COD achieved by the flow-proportional addition of flocculants is here crucial for an increased reduction of nitrogen. Normally, better than 60% reduction of Ntot (total nitrogen) and better than 98% reduction of Ptot (total phosphorus) is achieved.

An important part is to distribute the incoming water flow over the entire bed, which is achieved by pumping the water treated in the sludge separator out into the bed in portions, distributed over the 24 hours of the day.

The life of the soil bed or infiltration
A properly installed and dimensioned soil bed or infiltration built with bioModules has a very long life, 20-30 years or more?

We have opened ground-based facilities that have been in operation for more than 10 years, and found no tendency whatsoever for bed clogging. See video at www.baga.se.

We summarize as follows:
– An infiltration or soil bed constructed with bioModules and spreader plates works for a very long time provided that the instructions and dimensions are followed during
installation:
– Check that the sludge separator and subsequent infiltration or soil bed are well functioning.
– That the highest groundwater level is at most at least one meter below the spreader pipe, or at least 70 cm below the spreader plates.
– Sludge emptying according to the instructions.
– That the customer has carried out their own checks

A soil bed or infiltration made up of bioModules and spreader plates, to which the wastewater is pumped from a sludge separator with flow-proportional mixing of flocculants, is the simplest, safest and most economical way to solve a property’s sewage system.

Outgoing water always meets the Swedish Environmental Protection Agency’s requirements for “normal level”, and with the right dimensioning of the subsequent soil or infiltration bed, also “high level”. From a hygienic point of view, when flocculants have been mixed flow-proportionally in the sludge separator, satisfactory hygienization is normally achieved in the subsequent bed.

A land-based sewage treatment plant without any additional treatment step for phosphorus reduction is unlikely to achieve even 50% reduction due to leaching of bound phosphorus into the bed.
To ensure hygienization in the absence of additional treatment steps for phosphorus reduction, the top sand layer under the spreader plates under this condition should be of the fraction 0-8 mm.