The modern age

1997, Partial transformation of old physico-chemical section in a new biological section

With the aim to reduce the physico-chemical treatment in favour of biological ones, on 1997 new works for the conversion of physical-chemical plant took place. A new first biological treatment stage with high pollutant charge substituted the old physical-chemical with related economical and managerial advantages.

During functioning period some evidences emerged:

  • the first two sections (mechanical pre-treatment and first physico-chemical treatment) already used to remove:

COD                60-70% (from 10.000 to 3-4.000mgO2/L)

TSS                 90-100%

Sulphides        95-100%

Heavy metals   90%

  • the intermediate physico-chemical treatment didn't improve the eflfluent quality respect to the overflow stream from primary sedimentation
  • no significant amount of physico-chemical sludge was exctracted from the intermediate stage while from the first one about 75-80% of the total sludge production was exctracted
  • all these three sections didn't ensure the elimination of inhibitory substances which were organic and solubles, so they didn't precipitate and arrived to the biological stage
  • the biological section, sized for the treatment of about 54.000kgCOD/d and 5.400kgNH4+/d, during periods like from June to July and from October to November received higher amount ofCOD (70-80.000kgCOD/d with peaks up to  105.000kgCOD/d). As a consequence of those pollutants overload the biological process was stresses and COD and Ammonium concentrations rised up in the final discharge. In order to ensure the compliance with law limits, there was necessary a heavy tertiary treatment for the COD abatement and a final excess dosage of Sodium Hypochlorite for Ammonia abatement.
  • An optimal and reliable biological rtreatment required a COD income no over than 40-50.000kg/d

According to the aforementioned issues, in 1996 the intermediate physico-chemical section was converted in a first biological stage (or pre-biological) which would had allowed:

- to cut-off the incoming pollutant peaks;

- to demolish inhibitor substances such as organic dissolved chemicals which, because of theri solubility, didn't precipitate with chemicals agent used for the physico-chemical treatment

A research lasted few months in 1995 had showed that a first biological stage was able to reduce the COD quantity in the overflow stream from primary sedimentation stage, of about 50% and to improve the nitrification process due to the demolition of organic inhibitors.

 

Water section

Industrial Wastewater pre-treatments:

- gathering and mixing of all industrial wastewater in a common well

- pump lifting to mechanical pre-treatments

- fine screening by means of n.3 drum screens

- flow rate measurement

- flow spreading across two homogenisation basins with a volumetric capacity of 10.000mc each and where arrived also all plant internal recycles (from dehydration, thickening, drains, etc.)

- Sulphides oxidation within an homogenisation basin

Physico-chemical pre-treatment


- 2nd lifting section with Archimede's screw

- Ferric Chloride, Lime and anionic Polyelectrolyte addition

- spread across primary sedimentation tanks

- primary sedimentation n. 3 tanks of 960mc each.

Extracted physico-chemical sludge were transferred to n.2 circular thickeners of about 2.800mc each.
50-60% of the total amount of plant sludge production came from the primary physico chemical treatment.

Primary biological treatment (ex-second physico-chemical treatment):

- n. 1 biological oxidation basin of 7.000mc where pure oxygen was dosed

- n. 4 sedimentation tanks of 960mc each 

- recycle and purge of biological sludge.

Excess activated sludge were pumped into n.2 thickeners of 1200mc each (called biological thickeners); the dry solid content coming from this stage was about the 20-30% of the plant total solids production.

Municipal wastewater pre-treatments:

Incoming municipal wastewater, from the mixed sewerage of Santa Croce sull'Arno and from part of Fucecchio sewerage, was settled within the fifth sedimentation tank identical to the other four of the industrial biological sedimentation section. Primary sludge originating in this first stage was transferred to primary thickeners while the overflow stream was gathered to the industrial one before entering the second biological treatment

Second biological stage:

The mixed clarified sewage (industrial and municipal) enetred the second biological stage which didn't change anything.
Excess sludge was then transferred to the two biological thickeners. The dry solids evacuated as excess sludge from this section were about 10-20% of the plant total production.

Final physico-chemical tertiary treatment:

Overflow stream from sedimentation basins proceeded to the final physico-chemical treatment. The final discharge used to always comply with law limits except for Chlorides and Suplhates, for which a special regional derogation was still valid (see Table below).
As described before, whole sludge production of the WWTP went to their conditioning with ferric chlorides and lime before to be dehydrated due to the n.5 filter-presses.
After all these modifciations and updating of the pocess, about 120.000-130.000ton/year of dehydrated sludge was sent to landfill disposal.
During these years further experimentations were done with the aim to reduce the environmental impact of the process, specially regarding to the emission of smelly substances. The main activity regarded the collaboration with Solvay for sulphides abatement due to the hydrogen peroxides dosage.

Main advantages of hydrogen peroxide tilisation were:

- elimination of unpleasant odours emissions

- reduction of landfill disposal costs (no extra-sludge was produced because of the hydrogen peroxide use)

- elimination of pipes corrosion preoblems

- more economical than iron and lime treatment

- reduction of filter press equipment wear

- easy storage of raw materials and treatment management system

The hydrogen peroxide system was adopted since 1997 as a replacement for the ferric and lime treatment, enabling the remediation of unpleasant odours issue and the reduction of sludge production. The hydrogen peroxide pprocess was more economic and more permorming than the iron-lime one, furthermore its implementation included only one on-line ORP control connected to a dosing pump, ensuring the maximum outcome with the minimum investment. Although these benefits, after the plant modification and the introduction of the pre-biological section, the suphides oxidation was carried out within the homogenisation basin again, due to the use of pure oxygen.

Starting from 1997 increasing importance was assigned to the automation of the plant, as a result from those years a centralised infromatic system monitored and elaborated in real-time all plant techniocal data, from machine energy consumption to the discharge flow rate, all main process parameters, the weather condistions and odorous emissions, with the aim to have a better control on the environmental impact.

Among digital archives there were memorised years of managing data which allow everyone to verify the efficiency and the functioning of wastewater treatment systems.

Besides the automation of the monitoring system, manual controls continued to be pacticed by operators, main controlled parameters among treatment stages (entrance, homogenisation output, primary sedimentation output, first biological stage output, second biological stage output, final discharge), were:

•ph
•COD
•TSS
•N- NH4+
•N-NO2-
•N-NO3-
•Chlorides
•Sulphates
•Total Phosphorous
•Sulphides
•Liquid Dissolved Oxygen (LDO)
•Oxidation Reduction Potential (ORP)
•Thickeners and sedimentation tanks sludge level and concentration
•Microscopic and biological analysis of biomass 

 PARAMETER METRICSMOST FREQUENT VALUES AVERAGE VALUES LEGAL VALUES 
 pH 6,5-8,5 7,255,5 - 9,5
 COD mg/l 120 - 160 150160 
 SUSPENDED SOLIDS  mg/l 20 - 30  25  35 
 N-NH4+ mg N-NH4+/l 0,5 - 10  6 15 
 N-NO3- mg N-NO3-/l 8 - 1812  20
 SULPHIDES mg S=/l 001 
 SULPHATES  mg SO4=/l 1.300 - 1.6001.500  1.700
 CHLORIDES  mg Cl-/l2.200 - 2.8002.5003.800 

Whole plant analysis (excluded the single user discharge) were carried-out by the internal laboratory where worked n.4 technicians, which controlled the sludge quality as well as the chemicals one and they also characterised the tanker load. Between 1993 and 1994 the plant suffered relevant disfunction and Public authority in some cases intervened to stop the industrial activity because of the overcoming of discharge law limits. Beside this, in order to safeguard the environemnt and operators health, orders bannings were produced, also on the grounds of public demonstrations against unpleasant odours.
From 1997 to 2001 the process consolidated its treatment performances ensuring an overall abatement of pollutants of more than 98,5% (as COD reduction) and acquiring higher quality level for air emissions which permitted to gain also the Public Authority recognition. Some chemical such as sodium hydroxide, lime, and hydrogen peroxide have been using since those years with the only target to improve the environmental quality. Not only these chemicals are dosed within the plant but also along the sewerage system and the air treatment systems.